Messages in this thread Patch in this message | | | Date | Sat, 26 Mar 2005 12:34:26 +0100 | From | Willy Tarreau <> | Subject | Re: Linux 2.4.30-rc2 |
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Marcelo,
just another one and that's all. Zachary Amsden found an unconditional write to a debug register in the signal delivery path which is only needed when we use a breakpoint. This is a very expensive operation on x86, and doing it conditionnaly enhanced signal delivery speed by 33% for him.
His patch got merged in 2.6.10, and I've merged it a month ago in my local tree. Could we get it in 2.4.30, please ?
Thanks in advance, Willy
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I noticed an unneeded write to dr7 in the signal handling path for x86. We only need to write to dr7 if there is a breakpoint to re-enable, and MOVDR is a serializing instruction, which is expensive. Getting rid of it gets a 33% faster signal delivery path (at least on Xeon - I didn't test other CPUs, so your gain may vary).
Cheers,
Zachary Amsden zach@vmware.com
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Optimize away the unconditional write to debug registers on signal delivery path. This is already done on x86_64. Measured delta TSC for three paths on a 2.4GHz Xeon.
1) With unconditional write to dr7 : 800-1000 cycles 2) With conditional write to dr7 : 84-112 cycles 3) With unlikely write to dr7 : 84 cycles
Performance test using divzero microbenchmark (3 million divide by zeros):
With unconditional write: 7.445 real / 6.136 system 7.529 real / 6.482 system 7.541 real / 5.974 system 7.546 real / 6.217 system 7.445 real / 6.167 system
With unlikely write: 5.779 real / 4.518 system 5.783 real / 4.591 system 5.552 real / 4.569 system 5.790 real / 4.528 system 5.554 real / 4.382 system
That's about a 33% speedup - more than I expected; apparently getting rid of the serializing instruction makes the do_signal path much faster.
Zachary Amsden (zach@vmware.com)
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[hand-edited line numbers to match 2.4] --- linux-2.6.10-rc1/arch/i386/kernel/signal.c 2004-10-25 11:15:43.000000000 -0700 +++ linux-2.6.10-rc1-nsz/arch/i386/kernel/signal.c 2004-10-26 14:30:54.000000000 -0700 @@ -600,7 +600,9 @@ * have been cleared if the watchpoint triggered * inside the kernel. */ - __asm__("movl %0,%%db7" : : "r" (current->thread.debugreg[7])); + if (unlikely(current->thread.debugreg[7])) { + __asm__("movl %0,%%db7" : : "r" (current->thread.debugreg[7])); + } /* Whee! Actually deliver the signal. */ handle_signal(signr, &info, &ka, oldset, regs);
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