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.