I forgot to mention: the function in question is a simple random number generator. It only contains 32-bit integer math operations, and does not contain any loops. For this benchmark, the looping occurs at the Perl level, so alignment optimizations for looping would not be important here. (This lets me compare many different Perl-to-C function invocation approaches to assess their speed.)
On Tue, Dec 20, 2016 at 10:44 PM, David Mertens <[email protected]> wrote: > Discussion about alignment and execution speed for the Haskell compiler: > https://ghc.haskell.org/trac/ghc/ticket/8279 > > This discussion mentions why things should be aligned, and gives some > multi-byte no-ops that can be used for padding for aligned loops. > http://stackoverflow.com/questions/18113995/performance-optimisations-of- > x86-64-assembly-alignment-and-branch-prediction > > I came across a similar issue a few weeks ago, but I was able to "fix" it > by allocating more memory than I needed and then relocating to an address > within that allocation that was aligned to the start of a page. This seemed > to fix the problem back then, but this new flavor of alignment woes is > impervious to such a trick. > > David > > On Tue, Dec 20, 2016 at 10:29 PM, KHMan <[email protected]> wrote: > >> On 12/20/2016 10:17 PM, David Mertens wrote: >> >>> Hello Kein-Hong, >>> >>> I'm not convinced this is entirely an unpredictable hardware >>> issue. The reason is that I can easily create similar >>> functionality with gcc (the usual Perl XS module, the normal means >>> for writing a C-based extension) and it does not show these kinds >>> of cache swings. I think there is something gcc does while >>> producing its machine code that makes it less susceptible to cache >>> misses. (Well, there are lots of things it does, I'm sure.) I'm >>> hoping there's one or two simple things that gcc does which tcc >>> misses and could implement. >>> >>> Was the behavior observed with Lua noted when working with JIT? >>> >> >> I couldn't find the old posting but it was along the lines of benchmark >> variability due to memory layout, see "Mytkowicz memory layout". IIRC, the >> discussion was about a small benchmark Lua script running the interpreter, >> in one posting, changing an environment variable changed the program's >> total running time significantly, IIRC it was in the 20-50% range. The >> timings were done casually and nobody did detailed follow-up research. >> >> ... which of course are the same executables and is different from your >> case. Long day and all. But tcc is not much of an optimizing compiler, if >> the change caused register spilling in an inner loop it would hammer memory >> access and account for at least some of the effects... >> >> >> On Tue, Dec 20, 2016 at 9:05 AM, KHMan wrote: >>> >>> On 12/20/2016 9:16 PM, David Mertens wrote: >>> >>> Hello everyone, >>> >>> Reminder/Background: C::Blocks is my Perl wrapper around >>> my fork >>> of tcc with extended symbol table support. >>> >>> I've begun writing benchmarks to seriously test how C::Blocks >>> compares with other JIT and JIT-ish options for Perl. I've >>> noticed >>> a couple of situations in which slight modifications to >>> the code >>> cause a huge drop in performance. One benchmark went from >>> 370ms to >>> 5,000ms (i.e. 5 sec). >>> >>> The change to the code was so slight that I immediately >>> suspected >>> cache misses as the culprit. Running with linux's "perf" >>> command >>> gave proof of that (hopefully this format properly with >>> fixed-width characters): >>> >>> Fast Slow Significant >>> time (ms) 370 5022 ** >>> instructions 3.5B 3.5B >>> branches 640M 650M >>> branch-miss 687k 671k >>> dcache-miss 974k 71M ** >>> icache-miss 3.2M 83M ** >>> >>> By dcache-miss I refer to what perf calls "L1 dcache load >>> miss", >>> and by icache-miss I refer to what perf calls "L1 icache >>> load miss". >>> >>> I'm a bit confused on what would cause this sort of persistent >>> cache miss behavior. In particular, I've tried working >>> with highly >>> distinct strategies for managing executable memory, including >>> ensuring page alignment (wrong: it should be line-width >>> alignment >>> of 64 bytes). This fixed a similar issue previously >>> observed, but >>> didn't seem to improve the situation here. I used malloc >>> instead >>> of Perl's built-in memory allocator. I created a pool for >>> executable memory so that multiple chunks of executable >>> code would >>> all be written to the same page in memory. EVEN THIS did >>> not fix >>> this issue, which really surprised me since I would have >>> thought >>> adjacent memory would hash to different caches. >>> >>> I believe that what I've found is an issue with tcc, but I >>> haven't >>> golfed it down to a simple libtcc-consuming example. I can do >>> that, but wanted to see if anybody could think of an obvious >>> cause, and fix, without going to such lengths. If not, I >>> will see >>> if I can write a small reproducible example. >>> >>> >>> This kind of behaviour was discussed on the Lua list not long >>> ago. IIRC, for example changing environment variables changed >>> the way a program is loaded, and the timing changed. Probably >>> cache behaviour. It's like, what can we really benchmark anymore? >>> >>> When modern GHz parts have cache misses and need to access >>> main memory, they cause such train wrecks that everybody seems >>> to be moving or have already moved to neural network-based >>> (perceptron *cough*) branch prediction. >>> >>> So well, how do we scientifically or meaningfully benchmark >>> these days, that is the question... (especially for folks in >>> academic needing to justify benchmark results...) >>> >> >> >> -- >> Cheers, >> Kein-Hong Man (esq.) >> Selangor, Malaysia >> >> >> _______________________________________________ >> Tinycc-devel mailing list >> [email protected] >> https://lists.nongnu.org/mailman/listinfo/tinycc-devel >> > > > > -- > "Debugging is twice as hard as writing the code in the first place. > Therefore, if you write the code as cleverly as possible, you are, > by definition, not smart enough to debug it." -- Brian Kernighan > -- "Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it." -- Brian Kernighan
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