Messages in this thread | | | From | "Alexander van Heukelum" <> | Subject | Re: [v2.6.26] what's brewing in x86.git for v2.6.26 | Date | Thu, 17 Apr 2008 15:33:02 +0200 |
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On Thu, 17 Apr 2008 12:51:09 +0200, "Andi Kleen" <andi@firstfloor.org> said: > I think a realistic benchmark would be by running a real kernel > and profiling the input values of the bitmap functions and then > testing these cases. > > I actually started that when I complained last time by writing > a systemtap script for this that generates a histogram, but for some > reason systemtap couldn't tap all bitmap functions in my kernel and > missed some completely and I ran out of time tracking that down. > > My gut feeling is the only interesting cases are cpumask/nodemask sized > (which can be one word, two words but now upto 8 words on a NR_CPU=4096 > x86 kernel) and then 4k sized ext3/reiser/etc. block bitmaps. > > The generic version is out-of-line, > > while the private implementation of i386 was inlined: this causes a > > regression for very small bitmaps. However, if the bitmap size is > > a constant and fits a long integer, the updated generic code should > > inline an optimized version, like x86_64 currently does it. > > Yes it should probably. cpumask walks are relatively common.
Hi,
The version that is in x86#testing _will_ do this optimization. For 32 node SMP on x86_64 this results in:
<__first_cpu>: mov $0x20,%edx (inlined...) mov $0x100000000,%rax or (%rdi),%rax bsf %rax,%rax (... find_first_bit) cmp $0x20,%eax (superfluous paranoia...) cmovg %edx,%eax (... for broken find_first_bit) retq
and something similar for __next_cpu.
> I remember profiling mysql some time ago which did bad overscheduling > due to dumb locking. Funny was that the mask walking in the scheduler > actually stood out. No, i don't claim extreme overscheduling is an > interesting case to optimize for, but then there are more realistic > workloads which also do a lot of context switching. > > BTW if you do generic work on this: one reason the generated code for > for_each_cpu etc. is so ugly is that the code has checks for > find_next_bit returning >= max size. If you can generize the > code enough to make sure no arch does that anymore these checks > could be eliminated.
for_each_cpu code looks fine:
mov $cpumapaddress,%rdi callq <__first_cpu> jmp end_of_body start_of_body: ... end_of_body: mov $cpumapaddress,%edi ($mapaddress often cached in register) callq <__next_cpu> cmp $0x1f,%eax jle start_of_body
On the other hand it would be nice to change __first_cpu and __next_cpu into inline functions. If all implementations of find_first_bit and find_next_bit would reliably return max_size if no bits were found, that would be a good thing to do. The generic one does return max_size.
Greetings, Alexander
> -Andi -- Alexander van Heukelum heukelum@fastmail.fm
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