Aurelien Jarno wrote: > Laurent Vivier a écrit : >> Anthony Liguori wrote: >>> Laurent Vivier wrote: >>>> Avi Kivity wrote: >>>> >>>>> Aurelien Jarno wrote: >>>>> >>>>>> >>>>>> >>>>>>> What is your disk image file format, or are you using a partition? >>>>>>> >>>>>>> >>>>>> I am using a raw image file on an ext3 partition. >>>>>> >>>>>> >>>>>> >>>>>>> Do the results change (on kvm-14) if you pin the guest to a core with >>>>>>> 'taskset 1 qemu ...' >>>>>>> >>>>>>> >>>>>> Bingo. It now works even faster than kvm-13! >>>>>> >>>>>> real 0m22.307s >>>>>> user 0m13.935s >>>>>> sys 0m4.720 >>>>>> >>>>>> >>>>>> >>>>> I'm guessing this is due to the glibc aio implementation, which uses >>>>> threads instead of true aio. The threads may cause the vcpu to migrate >>>>> frequently from one code to another. >>>>> >>>>> There are two possible solutions: >>>>> >>>>> - use native aio from http://www.bullopensource.org/posix/. I think >>>>> the aio signal patches are not yet in, so this may not work. >>>>> - teach the scheduler about the cost of migrating vcpus >>>>> The first approach will solve itself eventually, though slowly if the >>>>> current slow rate of aio merging continues. We'll have to do the second. >>>>> >>>>> >>>> if you prefer the first one, Sébastien will release very soon aio patches >>>> for >>>> 2.6.20 with an up-to-date libposix-aio. >>>> >>> Hi Laurent, >>> >>> I gave that a shot a little bit ago. Ran into two problems. >>> >>> 1) Couldn't avoid linking to -lrt as QEMU uses time functions from it. >>> 2) While I could get things compiling (with patches), QEMU would SEGV >>> almost immediately. >>> >>> Could you guys maybe give compiling QEMU w/libposix-aio a shot? I'm >>> really interested to see if it makes a difference. >>> >>> Regards, >>> >>> Anthony Liguori >>> >>>> [advertising] Keep an eye on the bullopensource website. [/advertising] >>>> :-P >>>> >>>> Regards, >>>> Laurent >> OK, I didn' have time to test the performance of the result, but you can find >> attached some patches to enable libposix-aio with kvm-14. >> >> first take last patches for linux-2.6.20 and libposix-aio-.0.8.2 from >> website : >> >> http://sourceforge.net/projects/paiol >> >> if you are using AMD64, you must patch libposix-aio because there is a little >> problem remaining to detect lio_submit syscall (first attachment) >> >> then apply following patch to kvm-14 (second attachment). >> >> It works fine on my system except when I use "-hda /dev/sdb" : qemu crashes >> just >> after mounting filesystems when "init" tries to set kernel parameters with >> "sysctl" (I use a debian 4.0). If I boot in emergency mode, mounting manually >> filesystems and running manually sysctl, all works fine. It looks like a >> synchronization problem. >> >> There are remaining issues : libposix-aio uses kernel AIO, so files must be >> opened using O_DIRECT and buffers must be aligned. libposix-aio is able to >> manage other cases but this has a performance cost. >> >> Aurélien, do you have any time to test this on your system ? > > It took me some time to test it, I was not at home and I prefer to be > near the machine when I change the kernel!
thank you very much to have tested it : I was not able to reproduce your problem
with debian/linux guest on my intel xeon MP.
> I have just tried your patches, and I can say it works. I get the same
> times (minor measurement issues) as when using kvm without your patches
> and with taskset. Thanks a lot!
Good news.
> Would it be possible to merge the kvm part, and maybe to send it to the
> qemu mailing list?
I can try to send it to qemu mailing list but I think it is not useful for pure
qemu. It seems to improve performance only with kvm.
Avi, your opinion ?
> Regards,
> Aurelien
Regards,
Laurent
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