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?
On Sun, Feb 07, 2010 at 12:28:51PM -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.
Its not the primary reason, but the hardware virtualization extensions
of x86 processors support an intercept after a configured amount of
pause instructions were executed. This is used to detect spinning vcpus
where the lock-holder is scheduled out.
> 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%.
What benchmarks have you used for your measurements?
Joerg
On Sun, 07 Feb 2010 12:28:51 -0500
Michael Breuer <[email protected]> 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.
>
that's odd....
rep nop and pause ought to be the same...
--
Arjan van de Ven Intel Open Source Technology Centre
For development, discussion and tips for power savings,
visit http://www.lesswatts.org
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.
On 02/07/2010 03:08 PM, Michael Breuer wrote:
> On 2/7/2010 1:14 PM, Mike Galbraith wrote:
> , 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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Disregard case 2 - was missing -O3. With -O3 or -O2 rep;nop and pause
are identical. The interesting case is rep;pause which is different and
seems more efficient.
On 2/7/2010 4:15 PM, Michael Breuer wrote:
> On 02/07/2010 03:08 PM, Michael Breuer wrote:
>> On 2/7/2010 1:14 PM, Mike Galbraith wrote:
>> ...
>> Case1 - asm volatile("pause" ::: "memory");
>> 0000000000400480 <main>:
>> 400480: f3 90 pause
>> 400482: c3 retq
>> 400483: 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.
>>
> Disregard case 2 - was missing -O3. With -O3 or -O2 rep;nop and pause
> are identical. The interesting case is rep;pause which is different
> and seems more efficient.
Just to move away from this... totally perplexed, I retested a bit.
Seems something else had gone wrong causing me to try 'rep;pause' vs.
'pause'. The resulting opcode is f3 f3 90, as noted above.
I do see what seems to be a small but noticeable performance improvement
- no idea if there's a downside, and also no idea what f3 f3 90 does vs.
f3 90. Might be something interesting, or maybe not.
Test scenario:
Boot clean to single user mode. perform tiotest -8 five times.
%cpu is %usr + %sys as reported by tiotest.
Results:
Writes
pause: 1.14 sec; 72.01MB/sec; 322.44%cpu
rep;pause: 1.12 sec; 70.4MB/sec; 311.58%cpu
Random Writes
pause: 3.7 sec; 8.51MB/sec; 66.48%cpu
rep;pause 3.46sec; 9.04MB/sec; 72.34%cpu
Reads
pause: 11557.48MB/sec; 6040.74%cpu
rep;pause 11620.15MB/sec; 5974.90%cpu
Random Reads
pause: 11416.9MB/sec; 5330.50%cpu
rep;pause 11786.99MB/sec; 5118.66%cpu
Michael Breuer wrote:
> Just to move away from this... totally perplexed, I retested a bit.
> Seems something else had gone wrong causing me to try 'rep;pause' vs.
> 'pause'. The resulting opcode is f3 f3 90, as noted above.
>
> I do see what seems to be a small but noticeable performance improvement
> - no idea if there's a downside, and also no idea what f3 f3 90 does vs.
> f3 90. Might be something interesting, or maybe not.
Alignment? IOW what happens if you use eg "nop; rep; nop;" or "rep; nop; nop;"?