Hello
Here is the latest Caml Weekly News, for the week of May 12 to 19, 2009.
Archive: http://groups.google.com/group/fa.caml/browse_thread/thread/2a00d837b64efaf5/d1712ada74e6002f
Continuing the thread from last week, Xavier Leroy said:This is an interesting discussion with many relevant points being made. Some comments: Matteo Frigo: > Do you guys have any sort of empirical evidence that scalar SSE2 math is > faster than plain old x87? > I ask because every time I tried compiling FFTW with gcc -m32 > -mfpmath=sse, the result has been invariably slower than the vanilla x87 > compilation. (I am talking about scalar arithmetic here. FFTW also > supports SSE2 2-way vector arithmetic, which is of course faster.) gcc does rather clever tricks with the x87 float stack and the fxch instruction, making it look almost like a flat register set and managing to expose some instruction-level parallelism despite the dependencies on the top of the stack. In contrast, ocamlopt uses the x87 stack in a pedestrian, reverse-Polish-notation way, so the benefits of having "real" float registers is bigger. Using the experimental x86-sse2 port that I did in 2003 on a Core2 processor, I see speedups of 10 to 15% on my few standard float benchmarks. However, these benchmarks were written in such a way that the generated x87 code isn't too awful. It is easy to construct examples where the SSE2 code is twice as fast as x87. More generally, the SSE2 code generator is much more forgiving towards changes in program style, and its performance characteristics are more predictable than the x87 code generator. For instance, manual elimination of common subexpressions is almost always a win with SSE2 but quite often a loss with x87 ... Pascal Cuoq: > According to http://en.wikipedia.org/wiki/SSE2, someone using a Via C7 > should be fine. Richard Jones: > AMD Geode then ... Apparently, recent versions of the Geode support SSE2 as well. Low-power people love vector instruction sets, because it lets them do common tasks like audio and video decoding more efficiently, ergo with less energy. Sylvain Le Gall: > If INRIA choose to switch to SSE2 there should be at least still a way > to compile on older architecture. Doesn't mean that INRIA need to keep > the old code generator, but should provide a simple emulation for it. In > this case, we will have good performance on new arch for float and we > will still be able to compile on old arch. The least complicated way to preserve backward compatibility with pre-SSE2 hardware is to keep the existing x87 code generator and bolt the SSE2 generator on top of it, Frankenstein-style. Well, either that, or rely on the kernel to trap unimplemented SSE2 instructions and emulate them in software. This is theoretically possible but I'm pretty sure neither Linux nor Windows implement it. David Mentre: > Regarding option 2, I assume that byte-code would still work on i386 > pre-SSE2 machines? So OCaml programs would still work on those machines. You're correct, provided the bytecode interpreter isn't compiled in SSE2 mode itself (see below for one reason one might want to do this). However, packagers would still be unhappy about this: packaged OCaml applications like Unison or Coq are usually compiled to native-code (the additional speed is most welcome in the case of Coq...). Therefore, packagers would have to choose between making these applications SSE2-only or make them slower by compiling them to bytecode. Dmitry Bely: > [Reproducibility of results between bytecode and native] > I wouldn't be so sure. Bytecode runtime is C compiler-dependent (that > does use x87 for floating-point calculations), so rounding errors can > lead to different results. That's right: even though it stores all intermediate float results in 64-bit format, a bytecode interpreter compiled in default x87 mode still exhibits double rounding anomalies. One would have to compile it with gcc in SSE2 mode (like MacOS X does by default) to have complete reproducibility between bytecode and native. > Floating point is always approximate... I used to believe strongly in this viewpoint, but after discussion with people who do static analysis or program proof over float programs, I'm not so sure: static analysis and program proof are difficult enough that one doesn't want to complicate them even further to take extended-precision intermediate results and double rounding into account... To finish: I'm still very interested in hearing from packagers. Does Debian, for example, already have some packages that are SSE2-only? Are these packages specially tagged so that the installer will refuse to install them on pre-SSE2 hardware? What's the party line?Sylvain Le Gall then replied:
> The least complicated way to preserve backward compatibility with > pre-SSE2 hardware is to keep the existing x87 code generator and bolt > the SSE2 generator on top of it, Frankenstein-style. Well, either > that, or rely on the kernel to trap unimplemented SSE2 instructions > and emulate them in software. This is theoretically possible but I'm > pretty sure neither Linux nor Windows implement it. I was thinking (if it is possible) to use simple "function call" for doing float operation. This will be very inefficient, but will provide a very simple compatible layer. > To finish: I'm still very interested in hearing from packagers. Does > Debian, for example, already have some packages that are SSE2-only? > Are these packages specially tagged so that the installer will refuse > to install them on pre-SSE2 hardware? What's the party line? The more obvious package I see, is the linux kernel or the libc6: http://packages.debian.org/lenny/linux-image-2.6.26-2-486 http://packages.debian.org/lenny/linux-image-2.6.26-1-686-bigmem http://packages.debian.org/lenny/libc6 http://packages.debian.org/lenny/libc6-i686 AFAIK, there is no way for the package manager to do a real difference (no tag). However, the installer has some clue about which one to choose and install the best one for linux and libc6. Once installed, it is always updated in the good way, because the arch is embeded into the package name. I think linux and libc6 should be considered as exceptions, because they really provide an important benefit for overall optimization. For other package, if there is possible optimization, a version with and without optimization is embedded into the package and chosen at runtime. Example libavcodec provide i686 and i486 version: http://packages.debian.org/sid/i386/libavcodec52/filelist So in conclusion, there is always a "default" non SSE2 alternative for package that can provide an optimized version. I don't know any package that are SSE2-only. Im my opinion, Debian will probably refuse to ship a package that only provide SSE2-only version (but I am talking from my point of view).
Archive: http://groups.google.com/group/fa.caml/browse_thread/thread/632cf54350451087#
Vincent Balat announced:
Position available: research engineer
The PPS laboratory (http://www.pps.jussieu.fr) is recruiting an Research Engineer
for 2 years (22 months) possessing a good skill in (Ca)ML programming. The
position will be available in fall 2009.
Keywords: CDuce, Ocsigen, Web-services, Ocaml
Task:
The recruited person will be integrated to the ANR national project
Codex (http://codex.saclay.inria.fr) and will be asked to work on
the CDuce language [1], in particular:
- its use with the Ocsigen Web server [2]
- import export and composition of Web services
- integration with XProc [3]
- Maintenance of the CDuce distribution
Depending on its skill and background, the recruited person may be given
research oriented task such as but not limited to the extension of
CDuce with polymorphic functions.
About PPS:
PPS is a A-ranked CNRS laboratory of the University Paris Diderot Paris 7.
One of its main research topics is the the study of the programming languages
and distributed system and their logical foundations. The research activity is
coupled with an important software development activity spanning from the
Web (Ocsigen, CDuce, Xduce, Polipo) to parallel programming (CPC, Lwt, OcamlP3L), from networks (Babel) to the management of software packages (Edos, Mancoosi) and proof assistants (Coq).
Required skills:
- Expertise in OCaml programming
- Knowledge of Web standards
- Engineer or PhD degree (master is sufficient under some conditions)
Contacts:
Giuseppe Castagna and Vincent Balat:
{Giuseppe.Castagna || Vincent.Balat} @ pps.jussieu.fr
[1] http://www.cduce.org
[2] http://www.ocsigen.org
[3] http://www.xproc.org
Archive: http://groups.google.com/group/fa.caml/browse_thread/thread/a36a4b1f13b4ff56#
Mehdi Ben Soltane announced:Some of you already know MLstate, a young company which develops a new functional language for web development termed OPA (public release at the end of the year), and which codebase includes lots of OCaml. We are proud to announce that we seek to expand our nineteen-people team with three new positions: - *Product owner*. You manage one or several functional web-applications developed in-house, which means: You maintain the backlog, manage junior developers and/or trainees, you write OPA or OCaml code yourself and finally you meet the clients to make sure the apps suit their needs. - *Sysadmin*. You manage the internal and external networks of the company. You love to play with OS, you have deep knowledge of software tools, and you write functional applications to replace your initial quick and dirty Perl hacks… and you will know how to handle load. OPA will be of course the language of choice to develop nice web interfaces to do your job. - *Junior Application developer*. You love functional programming and already have a pretty good level with personal projects. Your job will be to develop webservices, with a functional style! If you think that one the positions could be yours, send your CV to careers at mlstate dot com to maybe join our friendly Paris-based office (no proxy at this time). We will make sure that the compensation package is interesting according to your profile.
Thanks to Alp Mestan, we now include in the Caml Weekly News the links to the recent posts from the ocamlcore planet blog at http://planet.ocamlcore.org/. FP-Syd #14.: http://www.mega-nerd.com/erikd/Blog/FP-Syd/fp-syd-14.html What do Haskellers have against exhaustiveness?: http://ocaml.janestreet.com/?q=node/64 A Polymorphic Question: http://alaska-kamtchatka.blogspot.com/2009/05/polymorphic-question.html libsndfile 1.0.20.: http://www.mega-nerd.com/erikd/Blog/CodeHacking/libsndfile/rel_20.html Okasaki's Random Access Lists: http://alaska-kamtchatka.blogspot.com/2009/05/okasakis-random-access-lists.html Presenting the 2009 JSSP projects: http://ocaml.janestreet.com/?q=node/63
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