Messages in this thread | | | Date | Sun, 07 Feb 2010 15:08:14 -0500 | From | Michael Breuer <> | Subject | Re: x86 - cpu_relax - why nop vs. pause? |
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On 2/7/2010 1:14 PM, Mike Galbraith wrote: > On Sun, 2010-02-07 at 12:28 -0500, Michael Breuer wrote: > >> I did search and noticed some old discussions. Looking at both Intel and >> AMD documentation, it would seem that PAUSE is the preferred instruction >> within a spin lock. Further, both Intel and AMD specifications state >> that the instruction is backward compatible with older x86 processors. >> >> For fun, I changed nop to pause on my core i7 920 (smt enabled) and I'm >> seeing about a 5-10% performance improvement on 2.6.33 rc7. Perf top >> shows time spent in spin_lock under load drops from an average of around >> 35% to about 25%. >> >> Thoughts? >> > /* REP NOP (PAUSE) is a good thing to insert into busy-wait loops. */ > > 00000000004004fc<rep_nop>: > 4004fc: 55 push %rbp > 4004fd: 48 89 e5 mov %rsp,%rbp > 400500: f3 90 pause > 400502: c9 leaveq > 400503: c3 retq > > 0000000000400504<pause>: > 400504: 55 push %rbp > 400505: 48 89 e5 mov %rsp,%rbp > 400508: f3 90 pause > 40050a: c9 leaveq > 40050b: c3 retq > > foo.c > > static inline void rep_nop(void) > { > asm volatile("rep; nop" ::: "memory"); > } > > static inline void pause(void) > { > asm volatile("pause" ::: "memory"); > } > > void main(void) > { > rep_nop(); > pause(); > } > > Interesting, and this got me thinking... and testing... I think there's an optimization issue with gcc:
First of all - a bit of background on how I got here:
After reading the Intel documentation, I tried replacing rep:nop with pause (in theory exactly what's shown above). The system hung on booting. I then tried replacing nop with pause (rep:pause) and the system booted. Using the above example, the opcode becomes f3 f3 90 vs f3 90 (rep nop).
Given the above compiler test case, this seemed odd, to say the least. So I played a bit more with gcc. Seems that the optimizer (-O3) is handling the *three*cases differently (objdump output)
Base code for all three cases (only change is the asm volitile line as shown for each case):
static inline void pause(void) { asm volatile("pause" ::: "memory"); }
void main(void) { pause(); }
Case1 - asm volatile("pause" ::: "memory"); 0000000000400480 <main>: 400480: f3 90 pause 400482: c3 retq 400483: 90 nop
Case2 - asm volitile("rep;nop" ::: "memory") Note: this didn't inline!
0000000000400474 <pause>: 400474: 55 push %rbp 400475: 48 89 e5 mov %rsp,%rbp 400478: f3 90 pause 40047a: c9 leaveq 40047b: c3 retq
000000000040047c <main>: 40047c: 55 push %rbp 40047d: 48 89 e5 mov %rsp,%rbp 400480: e8 ef ff ff ff callq 400474 <pause> 400485: c9 leaveq 400486: c3 retq 400487: 90 nop 400488: 90 nop 400489: 90 nop 40048a: 90 nop 40048b: 90 nop 40048c: 90 nop 40048d: 90 nop 40048e: 90 nop 40048f: 90 nop
Case3 - asm volitile("rep;pause" ::: "memory") 0000000000400480 <main>: 400480: f3 f3 90 pause 400483: c3 retq 400484: 90 nop _______ Note the difference between opcodes case 1 and case 3, and the mess made by the compiler in case 2.
As to benchmarks - I've checked a few things, no formal or lasting stuff... but striking at first glance:
1) At idle, perf top shows time spent in _raw_spin_lock dropping from ~35% to ~25%. 2) Running a media transcode (single core - handbrakecli): frame rate increased by about 5-10%. 3) During file-intensive operations (#2, above, or copying large files - ext4 on software raid6) - latencytop shows a decerase on writing a page to disc from about 120ms to about 90ms.
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