Messages in this thread | | | Date | Mon, 8 Aug 2011 11:40:28 -0400 | From | Jason Baron <> | Subject | Re: [PATCH] jump label: Reduce the cycle count by changing the link order |
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On Sat, Aug 06, 2011 at 12:10:09AM +0200, Peter Zijlstra wrote: > On Fri, 2011-08-05 at 16:40 -0400, Jason Baron wrote: > > In the course of testing jump labels for use with the CFS bandwidth controller, > > Paul Turner, discovered that using jump labels reduced the branch count and the > > instruction count, but did not reduce the cycle count or wall time. > > > > I noticed that having the jump_label.o included in the kernel but not used in > > any way still caused this increase in cycle count and wall time. Thus, I moved > > jump_label.o in the kernel/Makefile, thus changing the link order, and > > presumably moving it out of hot icache areas. This brought down the cycle > > count/time as expected. > > > > In addition to Paul's testing, I've tested the patch using a single > > 'static_branch()' in the getppid() path, and basically running tight loops of > > calls to getppid(). Here are my results for the branch disabled case: > > Those numbers don't seem to be pre/post patch, but merely > CONFIG_JUMP_LABEL=y/n so they don't tell us what the patch does. >
oops. I did record all that data, I just didn't include it :( So here it is:
jump label eanbled:
new makefile ordering:
Performance counter stats for 'bash -c /tmp/timing;true' (50 runs):
4,578,321,415 cycles ( +- 0.021% ) 3,969,511,833 instructions # 0.867 IPC ( +- 0.000% ) 751,633,846 branches ( +- 0.000% )
1.717374497 seconds time elapsed ( +- 0.021% )
old makefile ordering:
Performance counter stats for 'bash -c /tmp/timing;true' (50 runs):
4,623,129,746 cycles ( +- 0.015% ) 3,969,600,140 instructions # 0.859 IPC ( +- 0.000% ) 751,648,318 branches ( +- 0.000% )
1.734843587 seconds time elapsed ( +- 0.028% )
jump label disabled:
new makefile ordering:
Performance counter stats for 'bash -c /tmp/timing;true' (50 runs):
4,620,784,202 cycles ( +- 0.014% ) 4,009,564,429 instructions # 0.868 IPC ( +- 0.000% ) 771,654,211 branches ( +- 0.000% )
1.733853839 seconds time elapsed ( +- 0.031% )
old makefile ordering:
Performance counter stats for 'bash -c /tmp/timing;true' (50 runs):
4,623,191,826 cycles ( +- 0.009% ) 4,009,561,402 instructions # 0.867 IPC ( +- 0.000% ) 771,655,250 branches ( +- 0.000% )
1.734191186 seconds time elapsed ( +- 0.009% )
So, with jump labels enabled we get instructions and branches to fall even with the old Makefile ordering, but we don't get the corresponding fall in cycles/wall time, without the new Makefile ordering. This testing was done on a Kentsfield system.
> Anyway, should we put a comment in the Makefile telling us we should > keep jump_label.o last? >
Yes, I think that would be a good idea. I can re-post with the complete testing results and a Makefile comment, if we are ok with this change.
> Also, pjt mentioned on IRC that mucking about with link order is > something google is not unfamiliar with.. could we use some sort of > runtime feedback to generate linker layout maps or so? That seems like a > more scalable version than randomly mucking about with Makefiles :-)
Agreed. Definitely a good area to research. However, until we have that done, I think this patch makes sense.
Thanks,
-Jason
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