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Re: [Qemu-ppc] TCG PPC performance regression?

From: Mark Cave-Ayland
Subject: Re: [Qemu-ppc] TCG PPC performance regression?
Date: Tue, 06 Mar 2012 16:30:05 +0000
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20120207 Icedove/3.0.11

On 06/03/12 09:58, Jan Kiszka wrote:

Since I can reproduce this problem 100% of the time, I'm now trying to
verify whether or not this is an actual bug with just my setup, or a bug
in QEMU. I've had a look at the Jan's patch and can't see anything
immediately obvious that would cause this, so I'm wondering if this is
perhaps somehow compiler related? Any hints or reproduction of results
would be greatly appreciated.

Would I could imagine spontaneously is that my patch changed some memory
allocation ordering and, thus, maybe some alignment. But this is just a
blind guess.

Okay - so it looks like there are multiple bugs here, but it's primary cause is not your commit. The reason commit d5ab9713d2d4037fd56b0adddd26c8d4dc11cf09 struggles for any kind of performance is because of a separate ram_size bug which you fixed in commit 0ac543de94f4f5e7e79d59a56df60937cdd3cd24.

Unfortunately I haven't been able to do a comprehensive bisection because just after this point the memory API breaks PPC for quite some time, but I get the old (fast) speed at commit 2dd3022826bb1ced27d12493a8f1f4b6d4bc71b7 (+0ac543 backport) while the next commit I can find that works which is notably slower is 6c731dc2af1e80c410e8605098deb1ac16ab72f0.

I think I might try generating some gprof profiles to see if I can find out where the time is going.



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