[libvirt-users] dramatic performance slowdown due to THP allocation failure with full pagecache
Hi all, This is not really a libvirt issue but I'm hoping some of the smart folks here will know more about this problem... We have noticed when running some HPC applications on our OpenStack (libvirt+KVM) cloud that the same application occasionally performs much worse (4-5x slowdown) than normal. We can reproduce this quite easily by filling pagecache (i.e. dd-ing a single large file to /dev/null) before running the application. The problem seems to be that the kernel is not freeing (or has some trouble freeing) the non-dirty and (presumably immediately) reclaimable pagecache in order to allocate THPs for the application. This behaviour is also observable on regular bare-metal, but the slowdown is only 10-20% there - the nested paging of the guest really makes THP allocation important there (1). Both current CentOS and Ubuntu guests have the issue. Some more environmental context: the VMs are tuned (CPU pinning, NUMA topology, hugepage backing), so we normally see no difference between host and guest performance. After talking to many other colleagues in the HPC space it seems common that people setup their batch schedulers to put drop_caches between jobs, which can help to workaround this and other similar issues, but no-one has been able to explain why this is happening. Hopefully someone who knows about the guts of the Linux MM can shed some light...? (1) a related and possibly dumb question: in the case of a high-performance KVM where the guest is hugepage backed and pinned in host memory anyway, why do we still have a table based resolution for guest physical to host virtual address translation - couldn't this just be done by offset? -- Cheers, ~Blairo
On Tue, Nov 14, 2017 at 10:23:56AM -0700, Blair Bethwaite wrote:
Hi all,
This is not really a libvirt issue but I'm hoping some of the smart folks here will know more about this problem...
We have noticed when running some HPC applications on our OpenStack (libvirt+KVM) cloud that the same application occasionally performs much worse (4-5x slowdown) than normal. We can reproduce this quite easily by filling pagecache (i.e. dd-ing a single large file to /dev/null) before running the application. The problem seems to be that the kernel is not freeing (or has some trouble freeing) the non-dirty and (presumably immediately) reclaimable pagecache in order to allocate THPs for the application.
This behaviour is also observable on regular bare-metal, but the slowdown is only 10-20% there - the nested paging of the guest really makes THP allocation important there (1). Both current CentOS and Ubuntu guests have the issue. Some more environmental context: the VMs are tuned (CPU pinning, NUMA topology, hugepage backing), so we normally see no difference between host and guest performance.
After talking to many other colleagues in the HPC space it seems common that people setup their batch schedulers to put drop_caches between jobs, which can help to workaround this and other similar issues, but no-one has been able to explain why this is happening. Hopefully someone who knows about the guts of the Linux MM can shed some light...?
Transparent huge pages were never intended to provide any guarantees of performance for applications. They are essentially an optimization so that if there is free RAM, huge pages can be opportunistically given to a process even if it didn't ask for them. This gives KVM some improved performance in the default "out of the box" scenario where the user hasn't taken time to manually optimize setting. The kernel can't predict the future usage pattern of processes so it is not at all clear cut that evicting the entire pagecache in order to allocate more huge pages is going to be beneficial for system performance as a whole. IOW, if your application has a certain expectation of performance that can only be satisfied by having the KVM guest backed by huge pages, then you should really change to explicitly reserve huge pages for the guests, and not rely on THP which inherantly can't provide any guarantee in this area.
(1) a related and possibly dumb question: in the case of a high-performance KVM where the guest is hugepage backed and pinned in host memory anyway, why do we still have a table based resolution for guest physical to host virtual address translation - couldn't this just be done by offset?
Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
Thanks for the reply Daniel, However I think you slightly misunderstood the scenario... On 14 November 2017 at 10:32, Daniel P. Berrange <berrange@redhat.com> wrote:
IOW, if your application has a certain expectation of performance that can only be satisfied by having the KVM guest backed by huge pages, then you should really change to explicitly reserve huge pages for the guests, and not rely on THP which inherantly can't provide any guarantee in this area.
We already do this. The problem is not hugepage backing of the guest, it is THP allocation inside the guest (or indeed on a bare-metal host). The issue in the HPC world is that we support so many different applications (some of which are complete black-boxes) that explicit hugepage allocation for application memory is generally not viable, so we are reliant on THP to avoid TLB thrashing.
The kernel can't predict the future usage pattern of processes so it is not at all clear cut that evicting the entire pagecache in order to allocate more huge pages is going to be beneficial for system performance as a whole.
Yet the default behaviour seems to be to stall on fault, then directly reclaim and defrag in order to allocate a hugepage if at all possible. In my test-case there is almost no free memory, so some pagecache has to be reclaimed for the process, I don't understand why the THP allocation fails in this case versus when pagecache is lower though. -- Cheers, ~Blairo
On Tue, Nov 14, 2017 at 10:52:03AM -0700, Blair Bethwaite wrote:
Thanks for the reply Daniel,
However I think you slightly misunderstood the scenario...
On 14 November 2017 at 10:32, Daniel P. Berrange <berrange@redhat.com> wrote:
IOW, if your application has a certain expectation of performance that can only be satisfied by having the KVM guest backed by huge pages, then you should really change to explicitly reserve huge pages for the guests, and not rely on THP which inherantly can't provide any guarantee in this area.
We already do this. The problem is not hugepage backing of the guest, it is THP allocation inside the guest (or indeed on a bare-metal host). The issue in the HPC world is that we support so many different applications (some of which are complete black-boxes) that explicit hugepage allocation for application memory is generally not viable, so we are reliant on THP to avoid TLB thrashing.
Oh well THP usage inside the guest is then not really anything todo with virt, just a regular Linux questions, so not sure libvirt is the best place to ask. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
On 14 November 2017 at 10:56, Daniel P. Berrange <berrange@redhat.com> wrote:
Oh well THP usage inside the guest is then not really anything todo with virt, just a regular Linux questions, so not sure libvirt is the best place to ask.
True, I just hoped you or one of the other devs might have some insight on reclaim behaviour that would provide a clue. I guess I'll try a linux list. Any comment on this (which is virt related): (1) a related and possibly dumb question: in the case of a high-performance KVM where the guest is hugepage backed and pinned in host memory anyway, why do we still have a table based resolution for guest physical to host virtual address translation - couldn't this just be done by offset? -- Cheers, ~Blairo
On Tue, Nov 14, 2017 at 11:03:30AM -0700, Blair Bethwaite wrote:
On 14 November 2017 at 10:56, Daniel P. Berrange <berrange@redhat.com> wrote:
Oh well THP usage inside the guest is then not really anything todo with virt, just a regular Linux questions, so not sure libvirt is the best place to ask.
True, I just hoped you or one of the other devs might have some insight on reclaim behaviour that would provide a clue. I guess I'll try a linux list.
Any comment on this (which is virt related): (1) a related and possibly dumb question: in the case of a high-performance KVM where the guest is hugepage backed and pinned in host memory anyway, why do we still have a table based resolution for guest physical to host virtual address translation - couldn't this just be done by offset?
I don't know enough about KVM memory management to answer that. You probably need to ask on the KVM mailing list to get the expert answer. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
participants (2)
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Blair Bethwaite -
Daniel P. Berrange