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Michael Zucchi

 B.E. (Comp. Sys. Eng.)

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Saturday, 02 February 2019, 12:38

ez-alloc, libeze, that music player thing

So it's been a pretty long week of hacking. I went deep on the memory allocator and eventually got one debugged and working. I made some very silly mistakes which made what is fairly straightforward code a lot harder to get working than it should have been but that's the nature of the beast.

It grew into about 1000 lines w/ comments, but then again GNU libc malloc is 5000 in just the main file.

I'm kinda bummed out about the performance after all that though, which is leading down another hole ...

ez-alloc

So basically I started by taking a few ideas from GNU libc malloc:

Plus some other ideas and changes.

Before I got this working I decided to drop the per-size small lists. It was cluttering the code and I wanted to see how it worked without it.

I also added a huge allocator which uses a separate mmap for very large allocations, which is also what GNU libc does.

For accuracy testing I created a LD_PRELOAD module which replaced the memory allocation calls with my own and ran some binaries like ls or xterm. I can also use it to create a log of memory actions which I can replay so I can actually debug the thing.

For performance and overhead testing I have a small not-particularly representative test using libavl (it just allocates 1 larger block then thousands of 32-byte blocks). I read /usr/share/dict/words into an array. Then in order I insert them all into the tree, look them all up, then remove them. And another also not-particularly representative loop which frees and allocates random amounts.

Things that work well

I think the tiny allocator quite neat. It uses page alignment and a magic number to be able to allow ez_free() to quickly check whether an allocation is a tiny allocation and handle it appropriately.

It helps avoid fragmentation in the main memory and can save a lot of memory if there are many allocations of the same size.

It would be a big problem if a pointer was mis-identified as a tiny address when it wasn't. The possibility of this is not zero without additional checks, although very very unlikely.

Using ez-list and ez-tree makes the code relatively straightforward and easy to read. Having the same node pointers being used for either is a big plus - I can move nodes between trees or lists easily. List nodes can be removed without knowing the list header so I can have many lists but not care where they come from when performing a coalsecing (I use a single status bit to indicate wheather the block is in a list or the one tree). This makes the coalescing function 'just work' without having to worry about maintaining other state as mergable blocks are merged.

Things I really fucked up

Probably the worst was that I forgot to mask out the status bits of the size in the tree compare function. So ... nodes would go missing when I fiddled with the bits! These bits don't get changed very often while the node is in the tree so that lead to difficult to track down problems.

Idiot. Idiot.

gcc and weirdness

The C99+ aliasing rules broke certain sequences of ez-list calls as well. For example if you perform node.remove on the head of the list and then list.addhead, it would write the pointers back in the wrong order. Another subtle and hard to find bug and gcc didn't warn about it. I found a workaround using a soft barrier asm volatile ("" : : : "memory"); in node.remove but I don't really like it as I can't be sure other sequences might break. Using -fno-strict-aliasing works too of course. Maybe i'll just make the list and node pointers volatile, I don't think it will affect performance negatively doing this and the code pretty much requires them to behave that way.

I've had some other very strange behaviour with gcc as well, sometimes the optimiser makes radically different decisions for no obvious reason. It appears to do it even when I haven't made any code changes, but I don't think my memory is reliable to be absolutely sure on that.

Another waste of time?

The xterm test was quite interesting. With the tiny allocator enabled it quickly reaches a steady state with 21 pages of tiny allocations having over 50% occupancy. There are only 6 free large blocks, and 13 lost ones.

However pmap indicates more total memory is consumed vs glibc malloc which was a bit of a bummer.

For the performance tests I fix my CPU at a specific frequency to get more consistent and repeatable results.

The tree test runs about 10% faster than system malloc and saves roughly 1MB of maximum rss (/usr/bin/time), about 8.5MB vs 9.5MB. At least that's a winner. Actually if I remove the lookup loop so that the alloc code is more of the total time then the performance nearly equalises. So most of the performance improvement is down to allocation alignment and not using a tiny allocator. Which is quite useful all the same.

However the random test runs about 2-3x slower than system malloc and uses about the same total amount of memory. That was a real bummer and every time I compile the code it seems to get a bit slower (and use more peak memory to boot). But ... this is absolutely nothing like the allocation used in a real program such as xterm so I don't even know if it's worth worrying about. I should try with something more demanding but they likely already use pools or the like.

I suppose even if it is a bit slower it can be used as a low-overhead free-supporting pool allocator which might be useful.

I did some shitty profiling and found that the biggest cost was probably the tree manipulation during ez_alloc(). There are some simplish things I haven't tried that should help it.

For example when i split a free block I always remove the node and then re-insert it if there's any of it left (either the lost list or back into the tree). If I can determine that the new node has not changed ordering then it is possible to just replace the entry directly and save both the remove and insert.

When I coalesce nodes I find full consecutive runs and remove all the ones i've found as I go. Then insert the coalesced one back. It should be possible to do the same check here and save one of the removes and the insert in some cases but it is a bit more difficult to determine (maybe just find the largest node that belongs to the tree and try that?).

All non-tiny allocs trigger a coalesce and then search the free block tree. GNU malloc can sometimes use the most recently freed block here, which in this case would avoid the tree altogether.

Finally, I could always add back the specific-sized 'small' lists for recently freed blocks but I want to avoid that for various reasons.

I can't see these doubling the speed for this silly random test-case but I could be wrong.

I suppose I could also try other indexing structures as an alternative to the balanced tree such as a skip-list (really, a skip-index-array I guess). I've not used them before but I had a break yesterday and today it seems like it shouldn't be too difficult.

Update: 3/2/2019 I tried the skip-index idea. And lo, once I got it working the random test is now on par with GNU libc. I guess I will go with that then. I suppose then all i've really done is re-implement GNU libc malloc in the end; although the tiny allocator might benefit some usages. And well - the code is 1/5 the size and considerably more readable. And i'll wrap it up in a way that can be used as a pool allocator. I'm back to work this week so i'll have to give it a break for a while anyway; i'm also exhausted and need to get away from it. Spent some time in the garden today although I discovered the 47C head might've killed more roses out the front. Sigh.

Profiling

I haven't really looked into C profiling for years. Well apart from /usr/bin/time or gettimeofday() (although I just found clock_gettime()).

I suppose people use oprofile? I should at least look into it.

gprof is pretty much useless. Well I guess it did identify the tree manipulation as the major cost but looking at the code would tell you that anyway. It just doesn't sample often enough (i'm getting well under 1% sampling of running time).

Searches tend to turn up advice to use cachegrind, ... but I can't see anything resembling cycle counts in any of it's output. Ubuntu wont let me install kcachegrind because of some dependency issue (probably because i've blacklisted some snot like fucking-shit-audio) but I can't see how that can make up numbers which aren't there already

So ... I started working on my own as well. Yeah like that's a good idea! Ok, maybe it's really time to give it a break. It's really not something I need and i'm not sure -finstrument-functions is going to do a good enough job. I'll see i'll see.

I did snarf some more code out of ezesdk that I've added to libeze. I've got a very small ez-elf that lets you do basic but useful things like lookup function names.

That music player thing

So this has been a pretty extended foray well beyond the code I needed to implement a music player! I haven't touched that for weeks - on the other hand I have just left it running for those weeks and it's still going fine. The remote works well. I've got some tools to send commands around and it all just works.

The last time i mentioned it I was wondering how to do a menu but that night I came up with the obvious answer for a device with no display. Just make it talk!

So I poked around with espeak-ng for a while and fiddled with the player code so I can inject synthesised signal into the audio stream.

I got as far as implementing a say-this-song function. When you press the lips button it says the title and artist of the current song if they're available or just the filename if they're not. As I saw mention in a mailing list; it's quite a clear voice but unintentially quite hilarious at times! It also speaks too fast and doesn't pause anywhere nearly long enough for full stops but at least the former can be adjusted.

I then hit a bit of a design snag. The keyboard monitor takes the keys pressed and turns them into high level player commands. For example the cursor keys are fast-forward/rewind/next-track/previous-track. But during a menu these would change behaviour and be for navigating the menu.

So I wasn't sure where I should put the menu driver logic. If it was in the player then the keyboard module would just send through raw keypresses for the other keys and the player would behave differently depending on what's going on.

If it's in the keyboard module then it could route messages around to potentially other components and the player would implement higher level functions like 'enter menu mode', 'say this', 'preview that', 'rename playlist' and so on.

My initial ideas were a but muddled and the second case seemed a bit messy. But now I think about it again it makes a lot more sense and I can leave the player to playing music and it only really needs to maintain a 'playing' or 'not playing' state and do some low level stream pause management to make sure it all works with alsa.

Regardless, I do have a goal here and once I add a basic menu that would basically be a working player but i'm not sure when i'll get back to it.

Tagged hacking, libeze, playerz.
Tuesday, 29 January 2019, 14:47

Bugz

Fixed a couple of bugs in the blogz code to do with post urls, and added newer/older links on the individual post page.

I think I can now setup robots.txt so it doesn't over-index.

Should be able to just block '/tag' and '/blog?' I think. So it just indexes the front page and then anything /post/* it finds from it. Any cralwer should be able to find the whole blog from one post now they're linked.

Well i've put it in and i'll keep an eye on the logs to see what happens.

I can't add the post title to the prev/next post links yet but i've been working on a database based index which will allow it and some other useful things. I could just hack another index into the compiled-in indices but I think that path has outlived it's usefulness.

Tagged blogz, zedzone.
Tuesday, 29 January 2019, 09:39

Memory, IPC. Tradeoffs.

I've been playing around with a few ideas for libeze but it's not gotten anywhere particular yet. I'm getting stuck on the tradeoffs - even though they don't really matter much.

IPC - Serialisation

It started when I read a rant against serialsiation formats; this I agree with. An internal blob isn't that important but I thought i'd look at some to see if any where useful.

XDR

This is old and simple. It was written by a smart bunch from a smart company from a time when smarter decisions were being made about things like this. The biggest drawback is the big-endian nature, but I could always fudge it and just pass native-endian values and call it RDX or something.

I then had the bright idea of being able to decode structures without any additional memory - merely referencing the packet data. But this creates a couple of additional areas where it would diverge: strings would need to include their trailing nul byte to be useful, and embedded arrays might need specific alignment.

So really, it's not quite RFX anymore either. OTOH R is listed as `supporting' XDR but it's version is modified as well.

XDR is also not self describing but descriminated descriminated unions go a long way to handling versioning which is the main reason I might want such a feature.

ASN.1 BER/DER/CER/PER

There is actually some stuff I quite like about BER. It's self describing and relatively compact.

There's a lot that makes it a pain. A full implementation needs to implement a lot as it is vastly over-engineered; this means it is complex and likely to be buggy. REAL numbers are encoded in a way that needs a lot of fiddling about on either end.

It's not all wrong, but it's not too right either.

Protocol buffers

I've had the misfortune to have to deal with these for work.

The actual on-wire format isn't too bad. It's not self-describing but you can safely skip buts you don't understand. It's relatively compact. It's also a bit pointless.

The SDK is a complete pile of shit though. The Java implementation compiles gigantic code that runs fucking slow (List<Float>???). It moves all of the protocol variance decisions into the calling code so it's a messy pain to use as well: sort of negating one of the reasons to use a self-describing format in the first place.

For work I wrote my own decoder when I could - a couple of dozen lines of code. But some projects use such complex structures you end up with a gigantic library to decode it (supplied by protoc) and then a gigantic library to marshall it's output into something usable (that you write by hand).

JSON

It's just fucked. Even the same implementation can't decide how to canonically encode empty or single-element arrays, null or emptry strings, and so on.

It's not very compact and while it's realtively simple to parse it's not trivial either.

And then it's clumsy as fuck to use from any non-dynamic language. Getting fields you even know are there is a pain. Handling variances is worse.

CBOR (and half a dozen others)

Well this is just binary JSON. It fixes the end-to-end canonical encodingness and is arguably faster to parse but it's still going to be clumsy to use and not significnatly more compact except for arrays of small integers.

It also has some questionable design decisions and is already bloating with a dozen extension types - it seems like it wants to look as 'valid' as ASN.1/BER but with an even messier encoding scheme.

The whole "look we have a completely-defined 256 element jump table" is just a fucking weird thing to have in a serialisation format RFC. And nothing says simple like 256 right? What on earth is that even about?

SDXF

This is another RFC I came across along the way - not sure how as it just took me 10 minutes to re-find it.

Honestly it's a weird arsed format that looks like someone's internal stuff they decided to publish. I can't imagine anyone uses it.

I would note that with some small changes the meta-data I have can support pretty much all of these formats.

But I pretty much wasted a few hours of my life on this. After all that fucking around I would be inclined to just dump structures around if i didn't need strings! I have several desirable features which can't be covered by a single format anyway, although two would probably suffice.

e.g. here's some use cases.

Ok, maybe 3 formats. Although i'm not really interested in the 3rd at the moment.

The Local IPC case is basically a subset of XDR, aside from the aforementioned tweaks.

The rest? Plenty there but none are a great fit.

Memory

Memory allocation in C is simple enough but managing it can be a bit of a pain, hence pools.

Another issue is that a given implementation enforces certain constraints which can waste (significant) memory: for example on an amd64 platform, malloc() allocates at least 32-bytes for each allocation even if it's only a 5 byte string (this is is the minimum required to track free blocks). And every allocation also saves a size_t before the memory block so it knows how big it was when you call free: when you have many small objects of the same size this wastes a lot of memory. A more subtle problem is that memalign() must also save this size, so that if you start allocating many aligned structures you start to quickly get a holey memory.

Dealing with strings in general is a bit of a pain (and a source of many security issues) and only the GNU-sepcific obstack really provides any good support for it.

So I experimented with a few ideas.

High level internals of malloc() & free()

An implementation of this pair basically all work the same.

malloc will find a block of free memory big enough to hold the requested size plus space to hold that size. Some area of this block will be reserved for the allocation and the free block will either be reduced by the same amount, or removed (if the whole block is used).

free() will take the saved size and address and mark it as free. As part of this process it will find out if there are other free blocks immediately adjacent to this new free block and coalesce them into a single larger block if so. There are only 4 cases: isolated free, adjacent before, adjacent after, adjacent before and after.

The memory in the free block itself is used to hold the controlling structures required for the two functions. They cease to exist once the memory is allocated.

Additionally, for finding blocks there are two basic strategies:

first-fit

The free list is stored sorted by address. It is scanned from the start and the first block with enough memory to fit the request is used.

best-fit

The free list is sorted by size. It is scanned from a block which is larger-or-equal to the desired size and the first one is used.

Each has their benefits and trade-offs.

first-fit may be slower to allocate but will more effectively utilise memory - less total memory will be required. I did some basic and non-exhaustive testing that gave me this result. It can also be implemented with very simple code and the same list can be used for performing coalescing.

best-fit can be faster to allocate as you find the correct block directly. Based on my testing it requires more total memory. Additional data structures are required to implement the free() operation.

So finally, these choices any other indices arequired determine the data structures required to track and find the free blocks, and thus determine the minimum allocation size.

GNU libc malloc() uses a size_t for the block size, a double-linked list for the free list, and another size_t at the end of the free block for fast coalescing. So the minimum size of a free block (and hence the minimum allocation size) is 32 bytes on a 64-bit system 16 bytes on a 32-bit system. I think it uses best-fit.

AllocMem()/FreeMem()

The first memory allocator I knew was the one from AmigaOS via AllocMem(). This only has a single constraint in that the minimum allocation is 8 bytes. There is no fixed allocation overhead to store the allocated size as you need to also provide the size to FreeMem(). Free nodes are stored as a single-linked list.

It takes very little code to implement this and it utilises memory very well (it can't really be more utilised than 100%).

The drawback is that both allocation and deallocation are O(N) operations since both have to walk the free list. I guess 'slow' is relative as they did ok on a 7Mhz CPU with a global(!) shared memory list. But because one knew it was slow you wrote code to avoid it (SunOS also had a dreadfully slow allocator so it wasn't unique in this regard).

On memory constrainted systems I think it's still a useful technique. free() can also be accelerated somewhat by using a balanced tree by bumping up the minimum allocation.

ez-page-alloc

So the first idea I played with was a page allocator which would underly more specific algorithms. The idea would be that it would require the free size like FreeMem() and so allow pages to be allocated with no overhead and no wastage.

I didn't really get anywhere with this and then revisited it in a different way later on (ez-alloc-alloc).

ez-string-alloc

I diverted for a little while trying to work on a string allocator. It would take some number of pages as an allocation block. In this case there would be no way to free individual strings. But they wouldn't require any alignment or size overhead either.

Strings can be built a character at a time, and large strings are moved over to their own block.

Allocation can be made very fast by only searching the most recent block. Or it can be made more compact by searching all blocks. Or some trade-off in-between.

I think this could be the basis of a good no-free high performance pool allocator.

On the way I came across apr_pool and it's code is pretty ugly and I was suprised it didn't support free() of individual allocations, given how complex it is.

ez-fixed-alloc

I've written multiple memory allocators in the past and one of the easiest and most useful is an allocator for fixed-sized blocks. It was particularly important for glib container classes.

Free is trivial as you already know the size and never have to coalesce free blocks and you can just plop the memory into the head of a single-linked list.

Allocate is likewise as trivial as a single-linked list.

However you end up having to have multiple allocators for each size so you just end up with a lot of wasted space anyway.

Eh I dunno, i'm thinking about whether this one is useful at all.

ez-alloc-alloc

I had a look at an allocator that could support memalign() without wastage. And basically do-away with a need for a special page allocator.

This is basically a tree-based version of AlocMem()/FreeMem() but with a very large alignment. Because there is no size overhead all memory is used and allocating new page-aligned pages doesn't leave unfillable holes.

Performance problems with the algorithm would be mitigated by having fewer, larger allocations and leaving the internal allocations to other more specific algorithms.

Still I dunno, it's many drawbacks in this form but I like the idea. Probably some sort of external index (radix-tree) would be better and enforcing N*page sized allocations.

ez-pool-alloc

I tried to write an allocator that supported free but with the minimum overhead. I got it down to a minimum allocation size of 8 bytes (void *) with no overhead.

Basically I wrote another AllocMem()/FreeMem() but this time much closer to the original. To support the minimum allocation size of 8 bytes on a 64-bit platform the lsb of the link pointer is used to indicate whether the size is 8 bytes (no room for size) or larger (has size).

This still suffers from the same performance problems, although it can be mitigated by being used only for short-term memory or with few frees.

The Story So Far. A Big Waste Of Fucking Time.

In the end i'm not really sure I like anything I came up with.

I'll probably change the existing ezelib serialiser, and likely split the code up so that the description part is separate from the (multiple) encoding part(s). I'll probably have something XDR-like and another more compact one, maybe tagged. Each may not support all possible structures.

The memory stuff was really just fucking around; I don't remotely need any of this. But I will poke at it some more I guess. Probably a pageish allocator which manages system memory without allocation overhead (at point of allocation), then a couple of allocators which sit atop of it and can be used as pools. A general purpose one and a high-performance non-freeing one that also supports incremental string/structure creation.

The general purpose pool might just end up being very similar to malloc() but with the (rather useful) benefit of being able to be freed en-mass. I can possibly get the minimum allocation size down to 16 bytes although I will need to borrow some bits from ez-tree's ez_node to achieve this. I can possibly also get first-fit to work efficiently (order tree by size then address?). I think i'm going to have to save the size in allocations though (thus 8-byte allocation overhead): it's going to be needed a lot of the time and I can't do fast coalescing without it. If I have a particualrly fast page allocator i might be able to do something neat for small fixed-sized allocs and that would also cover the interesting memalign cases.

Ultumately they would also need valgrind instrumentation as well.

Probably back to work next week, should get away from the screen. Wheather's nice and warm but not too hot. Finally getting a few tomatoes and the sweet-corn is nearly there.

Tagged hacking, libeze.
Friday, 25 January 2019, 15:35

blogz - a personal blog engine

Time to announce the release of another new project - blogz.

Actually it's an old project, it is the software which is running the blog portion of this site. I mentioned months ago that I would get around to releasing it at some point and this is that point.

Go read the homepage for a quick summary. As usual the source has a decent README.

Tagged blogz, zedzone.
Thursday, 24 January 2019, 23:16

Well that was shit

It ended up hitting 46.6 - yep the hottest day on record - just as I left to hit the town. The town was dead. I spent most of the time drinking alone outside on the street although I used splashes of water to keep me cooler than I would have been otherwise.

Even the cockroaches were succumbing to the heat - well at least one expired while I was sitting there! So much for the nuclear apocalipse.

As a health exercise it was a complete failure and probably worse than just staying home.

At least it's supposed to cool down tomorrow for a few days although at 23:00 it's still about 35 so it's going to be a hot-arse night.

Tagged biographical.
Thursday, 24 January 2019, 15:43

Hell on Earth

Welcome to Adelaide, South Australia.

So the mercury just hit 46.6C, which would make it hottest day on record if that sticks as an official reading. Thats a prelimary number from the BOM and i'm not sure which site they use as the official Adelaide number (it was 47.7 in Kent Town). Not even 4pm yet so it might get higher although some clouds are rolling in and there's some `breeze' (more like a fan-forced oven).

Probably not the best day for it but I have to get out of the house for health reasons so time to go see what a burning city looks like first hand. Going to be a warm ride in.

Tagged biographical.
Thursday, 24 January 2019, 12:50

libeze - C utility library

This is announcing a new small project - libeze.

It is a small collection of C utilities as a static link library.

Goals:

Rather than copy the rest of it's current home page just go there and read it. There is much more in the README and source.

It's an offshoot (read: I got very distracted) of the mele music player i've been working on as well as some previous code from ezesdk.

Tagged code, hacking, libeze.
Saturday, 19 January 2019, 22:27

For the PlayerZ

Well i've been stuck in the house for various reasons so i've been doing a lot of solid work on the audio player software for the mele box.

It's not feature complete but it all works:

I'm using posix message queues for communicating between the processes with a simple C struct serialisation mechanism. I'm not using threads as yet, the player just runs as a single thread. This means it can block while reading from libavformat but it greatly simplifies the player logic.

At the moment i'm just running it on my 'workstation' so I don't know how much work - if any - will be required to get it going on the mele, or how it will handle the processes and so on. For example if the disk indexer will overload the cpu or i/o.

Anyway here is a high-level diagram of the processes and various bits that make it up. Sorry it's a bit shit, but well, openoffice draw is a fucking headache to work with.

For it to be feature complete there isn't much left. Some sort of playlist mechanism and/or other ways to navigate the files beyond disk+file-path order. Shuffle should be easy. Network streams would be nice.

With no user interface beyond 2 leds (and given they're in the same hole that just means 2 colours) it can't get too complicated without resorting to an additional service for external maniplation. But that is of course possible.

The things I want to look at are playlists and other ways to navigate the files (other than file-path order), and network streams.

Bored as Fuck

Haven't left the house for a full week at this point. I was sick for a few days but now i'm just miserable for other reasons. This stuff is keeping me well occupied but yeah it's kind of pointless. Even my knees fucking hurt.

I'm wondering whether i should just go back to work early since there's still a couple of weeks before I would otherwise. I don't think I could face it though.

I'm also a bit pissed off that I think I drowned and killed a chilli plant i've been growing for weeks just as it started to kick into gear. We had a couple of hot days and i overwatered it i think. Actually not much is growing very well this year and I can't seem to water it properly either. Too much or too little. Water is so expensive here.

Oh, I also wasted a couple of hours playing with yacy. The idea of decentralised internet services certainly appeals. But it was just slow and provided almost no relevant results to any search I tried - it just didn't work for me. Solr/Lucene just does a weighted sub-string index which isn't very sophisticated. It's also configured for some big server so running it on this little vps was difficult even after fighting with the complex configuration system.

I'm not eating much at least so maybe i can lose some of the weight I keep putting on.

Tagged hacking, linux, playerz.
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