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...?