Michael Zucchi

 B.E. (Comp. Sys. Eng.)


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Tuesday, 09 October 2018, 10:07

Other JNI bits/ NativeZ, jjmpeg.

Yesterday I spent a good deal of time continuing to experiment and tune NativeZ. I also ported the latest version of jjmpeg to a modularised build and to use NativeZ objects.

Hashing C Pointers

C pointers obtained by malloc are aligned to 16-byte boundaries on 64-bit GNU systems. Thus the lower 4 bits are always zero. Standard malloc also allocates a contiguous virtual address range which is extended using sbrk(2) which means the upper bits rarely change. Thus it is sufficient to generate a hashcode which only takes into account the lower bits (excluding the first 4).

I did some experimenting with hashing the C pointer values using various algorithms, from Knuth's Magic Number to various integer hashing algorithms (e.g. hash-prospector), to Long.hashCode(), to a simple shift (both 64-bit and 32-bit). The performance analysis was based on Chi-squared distance between the hash chain lengths and the ideal, using pointers generated from malloc(N) for different fixed values of N for multiple runs.

Although it wasn't the best statistically, the best performing algorithm was a simple 32-bit, 4 bit shift due to it's significantly lower cost. And typically it compared quite well statically regardless.

static int hashCode(long p) {
    return (int)p >>> 4;

In the nonsensical event that 28 bits are not sufficient the hash bucket index it can be extended to 32-bits:

static int hashCode(long p) {
    return (int)(p >>>> 4);

And despite all the JNI and reflection overheads, using the two-round function from the hash-prospector project increased raw execution time by approximately 30% over the trivial hashCode() above.

Whilst it might not be ideal for 8-bit aligned allocations it's probably not that bad either in practice. One thing I can say for certain though is NEVER use Long.hashCode() to hash C pointers!


I also tuned the use of synchronisation blocks very slightly to make critical sections as short as possible whilst maintaining correct behaviour. This made enough of a difference to be worth it.

I also tried more complex synchronisation mechanisms - read-write locks, hash bucket row-locks and so on, but it was at best a bit slower than using synchronize{}.

The benchmark I was using wasn't particularly fantastic - just one thread creating 10^7 `garbage' objects in a tight loop whilst the cleaner thread freed them. No resolution of exisitng objects, no multiple threads, and so on. But apart from the allocation rate it isn't an entirely unrealistic scenario either and i was just trying to identify raw overheads.


I've only started looking at the reflection used for allocating and releaseing objects on the Java side, and in isolation these are the highest costs of the implementation.

There are ways to reduce these costs but at the expense of extra boilerplate (for instantiation) or memory requirements (for release).

Still ongoing. And whilst the relative cost over C is very high, the absolute cost is still only a few hundred nanoseconds per object.

From a few small tests it looks like that maximum i could achieve is a 30% reduction in object instantiation/finalisation costs, but I don't think it's worth the effort or overheads.

Makefile foo

I'm still experiemnting with this, I used some macros and implicit rules to get most things building ok, but i'm not sure if it couldn't be better. The basic makefile is working ok for multi-module stuff so I think i'm getting there. Most of the work is just done by the jdk tools as they handle modules and so on quite well and mostly dicatate the disk layout.

I've broken jjmpeg into 3 modules - the core, the javafx related classes and the awt related classes.

Tagged java, jjmpeg, nativez.
Sunday, 22 April 2018, 19:13

FFmpeg 4.0

Just a short post about the latest FFmpeg release. I tried building jjmpeg 3.0.1 against FFmpeg 4.0 and it compiles cleanly with no warnings.

So I think it should be good enough to go ... but I realised I don't actually have anything handy already written to test it against right now so that's only a guess.

Once I do i'll bump the version and do another release. This is more or less what I had planned to do today but instead got tags working on this site instead.

Tagged java, jjmpeg.
Monday, 25 December 2017, 23:03

jjmpeg 3.0 released

Put enough together to push out a release of jjmpeg.

It ended up 1700 lines of Java, 2000 lines of C, and 300 lines of Perl.

Apart from supporting the latest version of FFmpeg (at least when I started a couple of weeks ago), it's smaller, cleaner, and more complete than any previous version. Having said that this is essentially just a beta release.

This one is licensed GNU General Public License Version 3 (or later).

I've kinda had enough for the moment so it's a pretty bare home page, but it's there.

Merry XMAS!

Tagged java, jjmpeg.
Friday, 22 December 2017, 10:54

damned enums

Been a long week but i'm finally done with work for another year. Although it's mostly a long week because of the late nights working on jjmpeg ...

One of the things I did was fill out/sync up the important enums - AVCodecID, AVPixelFormat, AVSampleFormat, and so on. Previously the pixel format and sample formats were also Java enums - which can be convenient at times and provides some more (albeit much much overvalued) 'type safety'.

This was fairly easy because the PixelFormat was a simple densely ordered C enum so i could map between the two with a simple +-1. Unfortunately someone decided to add a big hole in the middle of it sometime between 0.10 and 3.4, ... so I gave up and just converted it to a class holding static final int's, and to make it consistent I did that with the other enumerations as well. It doesn't really make the classes any harder to use and improves the class size and memory footprint. I just added some methods to access libav*'s metadata information so I can still map between string representations and so on.

I had to add a small compilation stage which extracts the enums from the header files and converts them to a C file which when compiled and run produces the Java source ... this seemed the absolute shortest path to ensuring I got accurate numbers based on the ffmpeg build configuration.

So after about a weeks worth of solid work it's grown somewhat (about 2KLOC Java, and 2KLOC C, counting lines with ";{}") and the TODO list is getting pretty short.

I would like to clean up the exception design a bit - unfortunately i'm just not very good at that (who is?) but i'd like to get better. The build system is clean and simple but could be improved and needs to include the aforementioned enum stuff, a dist target and versioning. Logging would be nice (both redirect ffmpeg to java.util.logging and some for jjmpeg itself). JJMediaWriter? Fix the license headers, add at least a README.

Not today though, today I drink.

Tagged java, jjmpeg.
Tuesday, 19 December 2017, 20:26

jjmpeg, jni, javafx

So I guess the mood took me, I somehow ended poking away until the very late morning hours (4am) the last couple of nights hacking on jjmpeg. Just one more small problem to solve ... that never ended. Today I should've been working but i've given up and will write it off, it's nearly xmas break anyway so there's no rush, and i'm ahead of the curve anyway.


I got this ported over and playing video fairly easily, and then went through on a cleanup spree. I removed all the BufferedImage, multi-buffering, and scaling stuff and a few other experiments which never worked. Some api changes allowed me to consolidate more code into a base class, and some changes to AVStream necessitated a different approach to initialising the AVCodecContext (using AVCodecParameters). I made a few other little tweaks on the way.

The reason I removed the BufferedImage code is because I didn't want to pollute it with "platform specific" code. i.e. swing, javafx, etc. I've moved that functionality into a separate namespace (module?).

My first cut just took the BufferedImage code and put it into another class which provides the functionality by taking the current AVFrame from the JJMediaReader video stream. This'll probably do but when working on similar functionality for JavaFX I took a completely different approach - implementing a native PixelReader() so that the native code can decide the best way to write to the buffer. This is perhaps a little more work but is a lot cleaner to use.


jjmpeg1 lets you scale images 'directly' to/from primitive arrays or direct ByteBuffers in addition to AVFrame. Since they have no structure description (size, format), this either has to be passed in to the functions (messy) or stored in the object (also messy). jjmpeg1 used the latter option and for now I simply haven't implemented them.

The PixelReader mentioned above does implement it internally but for code re-use it might make sense to implement them with the structure information as explicit parameters, and use higher level objects such as PixelReader/Writer to track such information. On the other hand the native code has access to more information so it also makes sense to leave it there.

I went a bit further and created a re-usable super-class that does most of the work and toolkit specific routines only have to tweak the invocation. This approach hides libswscale behind another api. The slice conversions don't work properly but they're not necessary.


So far I had public constructors and `finalisers' because otherwise the reflection code failed. That's a bit too ugly (and `dangerous') so I made them private. The reflection code just had to look up the methods and set them Accessible.

    Constructor cc = jtype.getDeclaredConstructor(Long.TYPE);


    return cc.newInstance(p);

Whilst working on JJMediaReader I hit a snag with the issue of ownership. In most cases objects are either created anew and released (or gc'd) by the Java code, or are simply references to data managed elsewhere. I was addressing the latter problem by simply having an empty release() method for the instance, but that isn't flexible enough because some objects are created or referenced the the context determines which.

So I expanded the Java-side object tracking to include a `refer' method in addition to the 'resolve' method. `resolve' either creates a new instance or returns and existing one with a weak-reference object which will invoke the static release method when it gets finalised. `refer' on the other hand does the same thing but uses a different weak-reference object which does nothing.

I then noticed (the rather obvious) that if an object is created, it can't possibly 'go away' from the object tracking if it is still alive; therefore the `resolve' method was doing redundant work. So I created another `create' method which assumes the object is always a new one and simply adds it to the table. It can also do some checking but i'm pretty sure it can't fail ...

If on the other hand the underlying data was reference counted then the `resolve' method would be useful since it would be possible to lookup an existing object despite it being `released'. So i'll keep it in CObject.

As part of this change I also improved CObject in other ways.

I was storing the weak reference to the object itself inside the object so I could implement explicit release and to avoid copying the pointer. I removed that reference and only store the pointer now. The WeakReference it already tracked in a hash table so I just look it up if I need it. This lets me change the jni code to use a field lookup rather than a function call to retrieve it (I doubt it makes much perf difference but I will profile it at some point).

I also had some pretty messy "cross-layer" use of static variables and messy synchronisation code. I moved all map references to outside of the weak reference routine and use a synchronised map for the pointer to object table.

For explicit release I simply call .clear() and .enqueue() on the WeakReference - which seems to do the right thing, and simplifies the release code (at least conceptually) since it always runs on the same thread.

Tagged hacking, java, javafx, jjmpeg.
Sunday, 17 December 2017, 18:24

jjmpeg & stuff

Well for whatever reason I got stuck into redoing jjmpeg and seem to have written most of the code (90%?) after a couple of weekends. It was mostly mandraulic and a bit tedious but somehow surprisingly relaxing and engaging; a short stint of unchallenging work can be a nice change. A couple of features are still missing but the main core is done.

Unfortunately my hope that the ffmpeg api was more bindable didn't really pan out but it isn't really any worse either. Some of the nastiest stuff doesn't really need to be dealt with fortunately.

I transformed most of the getters and setters into a small number of simple macros, and thus that part is only about as much work as the previous implementation despite not needing a separate compilation stage. I split most of the objects into separate files to make them simpler to maintain and added some table-based initialisation helpers to reduce the source lines and code footprint.

It's pretty small - counting `;' there's only 750 lines of C and 471 lines of Java sources. The 0.x version has 800 lines of C and 900 lines of Java, a big portion of which is generated from an 800 line (rather unmaintainable) Perl script. And the biggest reduction is the compiled size, the jar shrank from 274KB to 73KB, with only a modest increase from 55KB to 71KB in the (stripped) shared library size (although the latter doesn't include the dvb or utility classes).

There's still a lot of work to do though, I still need to test anything actually works and port over the i/o classes and enum tables at the least, and a few more things probably. This is the boring stuff so it'll depend on my mood.

Fuck PCs

In other news I finally killed my PC - I tried one more time to play with the BIOS and after a few updates it got so unstable it just crashed during an update and bricked the motherboard. Blah. I discovered I could order a new BIOS rom so i've done that and i'll see if i can recover it, otherwise I might get another mobo if I can still get AM2+ boards here, or just get another machine. I'll probably look into the latter anyway as it's always been a bit of a hassle (despite working flawlessly when it does and it's a very nice small machine.

Tagged java, jjmpeg, rants.
Friday, 08 December 2017, 19:46


Well i've had reason to visit jjmpeg again for something and although it's still doing the job, it's a very very long way behind in version support (0.10.x?). I've added a couple of things here and there (recently AVFormatContext.open_input so I could open compressed webcam streams) but i'm not particularly interested in dropping another release.

But ... along the way I started looking into writing a new version that will be up to date with current ffmpeg. It's a pretty slow burner and i'm going to be pretty busy with something (relatively interesting, moderately related) for the next couple of months.

But regardless here are a few what-if's should I continue with the effort.

Does anyone else care?

Tagged java, jjmpeg.
Wednesday, 06 May 2015, 21:48

post google code post

Well nobody bothered to comment about the stuff i removed from google code apart from the one lad or lass who lamented the loss of some javafx demos.

I had comments open+moderated for a few weeks but got hit by spammers a couple of days ago so had to go back to id+moderated. Maybe something got lost in those 500 bits of snot but i don't think so. The spam was quite strange; most mentioned web sites but didn't provide links or weren't very readable so i'm not sure what the point was. Perhaps they're just fishing for open sites or naive moderators they can then exploit. Like the "windows computer department" that keeps calling and calling hoping i'll not tell them to fuck off every time (sigh, no i don't normally say that although i would tonight).

I've still got the subversion clones but i'm not inclined to do much with any of it for the forseeable future and i'm not even sure if i'm going to continue publishing other bits of code i play with going forward.

Desktop Java, OpenCL, ARM assembly language; these things are just not very common in the Free Software world. Server Java is pretty common but that's just, well, `open sauce' companies sharing costs and not hobbyists. So i think all i'm really doing is providing hints or solutions for some student's homework or help for graduate programmers to keep their jobs. And even then it's so niche it wouldn't be many, if any.

As an example of niche, I was looking up some way to communicate with adobe photoshop that doesn't involve psd format and one thing i came across was someone linking to one of my projects for some unfinished experiments with openraster format - on the first page of results. This happens rarely but still too often. Of course it could just be the search engine trying to be smart and tuning results to the user, which is a somewhat terrifying possibility (implications beyond these types searches of course). FWIW I came to the conclusion photoshop is just one of those proprietary relics from the past which intentionally refuses to support other formats so it's idiot users can continue to be arse-reamed by its inflated price.

It's just a hobby

As a hobby i have no desire to work on larger projects of my own or other established projects in my spare time. Occasionally i'll send in a patch to a project but if they want a bunch of fucking around then yeah, ... naah. In hindsight i somewhat regret how we did it on evolution but i think i've mentioned that before. Neither do i need to solicit work or build a portfolio or just gain experience.

I'm not sure how many hobbyists are around; anyone with remotely close to enough skill seems to be jumping into the wild casinos of app-stores or services and expecting to make billion$ and not just doing it for the fun of it. Some of those left over just seem to be arrogant egotistical fuckwits (and some would probably think the same of me). Same as it ever was I guess.

I suppose I will continue to code-drop even if it's just out of habit.

For another hobby I made kumquat marmalade on the weekend. Spent a couple of hours in the sun slicing the tiny fruit and extracting seeds (2-3 cups worth of seeds) and cooked it the next day. Unfortunately after all that effort it looks like it wasn't cooked quite enough and it probably wont set - it's a bit runny but at least it tastes good. Not sure what i'll do with 2-odd litres of the stuff though.

Tagged beagle, code, imagez, jjmpeg, mediaz, parallella, pdfz, puppybits, readerz, socles, videoz.
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