Re: [Testperf-general] Re: First set of OSDL Shared Memscalability results, some wierdness ...

Josh Berkus <josh(at)agliodbs(dot)com> writes:
> First off, two test runs with OProfile are available at:
> http://khack.osdl.org/stp/298124/
> http://khack.osdl.org/stp/298121/

Hmm. The stuff above 1% in the first of these is

Counted CPU_CLK_UNHALTED events (clocks processor is not halted) with a unit mask of 0x00 (No unit mask) count 100000
samples % app name symbol name
8522858 19.7539 vmlinux default_idle
3510225 8.1359 vmlinux recalc_sigpending_tsk
1874601 4.3449 vmlinux .text.lock.signal
1653816 3.8331 postgres SearchCatCache
1080908 2.5053 postgres AllocSetAlloc
920369 2.1332 postgres AtEOXact_Buffers
806218 1.8686 postgres OpernameGetCandidates
803125 1.8614 postgres StrategyDirtyBufferList
746123 1.7293 vmlinux __copy_from_user_ll
651978 1.5111 vmlinux __copy_to_user_ll
640511 1.4845 postgres XLogInsert
630797 1.4620 vmlinux rm_from_queue
607833 1.4088 vmlinux next_thread
436682 1.0121 postgres LWLockAcquire
419672 0.9727 postgres yyparse

In the second test AtEOXact_Buffers is much lower (down around 0.57
percent) but the other suspects are similar. Since the only difference
in parameters is shared_buffers (36000 vs 9000), it does look like we
are approaching the point where AtEOXact_Buffers is a problem, but so
far it's only a 2% drag.

I suspect the reason recalc_sigpending_tsk is so high is that the
original coding of PG_TRY involved saving and restoring the signal mask,
which led to a whole lot of sigsetmask-type kernel calls. Is this test
with beta3, or something older?

Another interesting item here is the costs of __copy_from_user_ll/
__copy_to_user_ll:

36000 buffers:
746123 1.7293 vmlinux __copy_from_user_ll
651978 1.5111 vmlinux __copy_to_user_ll

9000 buffers:
866414 2.0810 vmlinux __copy_from_user_ll
852620 2.0479 vmlinux __copy_to_user_ll

Presumably the higher costs for 9000 buffers reflect an increased amount
of shuffling of data between kernel and user space. So 36000 is not
enough to make the working set totally memory-resident, but even if we
drove this cost to zero we'd only be buying a couple percent.

regards, tom lane