Re: Android performance drop moving from LuaJIT-2.0.1 -> LuaJIT-2.0.2 ?
- From: Mike Pall <mike-1309@xxxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Wed, 4 Sep 2013 19:47:36 +0200
Kriss@xxxxxxxx wrote:
> I seem to be experiencing a huge lua speed drop when using LuaJIT-2.0.2 on
> android, code that was previously clocking in at around 5ms now take 100ms
>
> Dropping back to LuaJIT-2.0.1 fixes the problem.
>
> Running LuaJIT-2.0.2 with "jit.off()" fixes the problem.
That's rather strange ...
> So it does seem to be something to do with a change in the JIT code.
Here's the only possibly relevant change between 2.0.1 and 2.0.2:
commit 3e286c1e72651f91a2f4ca5b1fa2f366ac4cd0fa
Date: Thu Feb 28 17:51:16 2013 +0100
ARM: Fix cache flush/sync for exit stubs of JIT-compiled code.
diff --git a/src/lj_asm_arm.h b/src/lj_asm_arm.h
index afb1054..196f797 100644
--- a/src/lj_asm_arm.h
+++ b/src/lj_asm_arm.h
@@ -91,6 +91,7 @@ static MCode *asm_exitstub_gen(ASMState *as, ExitNo group)
*mxp++ = group*EXITSTUBS_PER_GROUP;
for (i = 0; i < EXITSTUBS_PER_GROUP; i++)
*mxp++ = ARMI_B|((-6-i)&0x00ffffffu);
+ lj_mcode_sync(as->mcbot, mxp);
lj_mcode_commitbot(as->J, mxp);
as->mcbot = mxp;
as->mclim = as->mcbot + MCLIM_REDZONE;
Try to remove the lj_mcode_sync line from 2.0.2, rebuild and test
again. If it goes away, we'll have to investigate further. Because
the sync is really needed as it was added due to a (rare) crash.
OTOH, if this is really the problem, then it would indicate your
code is generating traces at a very high rate. Which means you
should look at the output of -jv or the equivalent:
require("jit.v").start("/sdcard/wherever/my_dump.txt")
For this to work you'll need to install or bundle src/jit/v.lua
and src/jit/vmdef.lua.
> This also seems to be something to do specifically with Android rather than
> ARM, I'm also running luajit on the Raspberry Pi (although built with
> different flags) and not experiencing the same problem.
>
> Currently testing primarily on a Nexus7.
The RPi is an ARMv6 with a tiny cache, where too frequent cache
flushes would have little consequences. The Nexus 7 (2012) is a
quad-core Cortex-A9 which might be rather sensitive to that.
> This is part of a reasonably large project and it is of course possibly I
> have some other strangeness somewhere that is triggering the problem. Still
> the moving from 2.0.1 to 2.0.2 is my biggest clue on where the problem is
> so I'm hoping to try and work out exactly what that change was.
If the above change isn't the culprit, you could do a git bisect
until you find the commit that triggers it. A bit tricky to use
that for a large project, though.
--Mike
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