On 05.01.2011, at 20:54, Andrea Arcangeli wrote: > Hello everyone, > > On Wed, Jan 05, 2011 at 08:44:38PM +0100, Alexander Graf wrote: >> >> On 05.01.2011, at 19:02, Michael Roth wrote: >> >>> On 01/05/2011 09:10 AM, Andrea Arcangeli wrote: >>>> The bug is still there so I rediffed the old patch against current >>>> code. >>>> >>>> On a related topic: could somebody give me advice on how to implement >>>> a command line (command line seems enough, the other option would be >>>> monitor command) to make the MADV_MERGEABLE conditional? I got KSM on >>>> THP working fine but KSM may decrease performance by increasing the >>>> number of copy on write and by splitting hugepages, so we'd like to be >>>> able to turn off KSM on a per-VM basis (not on the whole host, which >>>> of course we already can by setting /sys/kernel/mm/ksm/run to 0) so >>>> that high perf VMs will keep running at maximum speed with KSM off but >>>> others may still benefit from KSM. For that I need to make the below >>>> MADV_MERGEABLE madvise conditional to something and the code itself >>>> will be trivial, we've just to converge on a command line option >>>> (hopefully quickly ;). >>> >>> There was a -mem_prealloc option added a while back to set MAP_POPULATE on >>> memory mapped in via the -mem-path option. So an analogous -mem_nomerge >>> option or something along that line seems reasonable for conditionally >>> unsetting QEMU_MADV_MERGEABLE. >>> >>> And for consistency you should probably make both your proposed changes for >>> -mem-path'd memory as well. >> >> Why not clean up all that mess and introduce a new -mem option that would >> just take all of the several options as parameters? >> >> -mem size=512,populate=on,ksm=off >> >> and default -m to something reasonable with the new syntax. > > I'm neutral... so feel free to decide what I should implement ;). > > One comment on combining -m ksm=off (or -mem_nomerge) with > -mem-path. It seems unnecessary because ksm can't be turned on on > VM_HUGETLB vmas (MADV_MERGEABLE will return -EINVAL) and mem-path only > makes sense if used in combination with hugetlbfs (which sets > VM_HUGETLB of course).
Sure, not all combinations make sense. But "-mem size=1G,path=/dev/shm/vm1.ram,populate=on" would make sense, no? TPH should go along the same lines here too. It'd just be a flag "tph" that defaults to on if available. That way we could also do all the sanity checks in a single place. I really like the idea of combining memory management command line parameters into a single option :). In the end I'd assume it's Anthony's call though. Alex
