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? David On Tue, Dec 20, 2016 at 9:05 AM, KHMan <[email protected]> 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
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