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SubjectRe: [RFC 0/5] disable pcplists during page isolation
On 2020-09-09 12:54, Vlastimil Babka wrote:
> Thanks! I expect no performance change while no isolation is in
> progress, as
> there are no new tests added in alloc/free paths. During page isolation
> there's
> a single drain instead of once-per-pageblock, which is a benefit. But
> the
> pcplists are effectively disabled for the whole of online_pages(),
> offline_pages() or alloc_contig_range(), which will affect parallel
> page
> allocator users. It depends on how long these operations take and how
> heavy the
> parallel usage is, so I have no good answers. Might be similar to the
> current
> periodic drain.

I have seen some systems taking quite some time when offlining sections
due to the migration of
the respective pages not being that smooth and having do_migrate_range
to do some spins.
But to be fair, online_pages and offline_pages are not routines that get
called that often, and we would be safe to assume that memory-hotplug
operations are not constantly happening, but are rather one-offs
operations.

I am not sure about Xen and HV, IIRC Xen was using online_pages and
offline_pages routines to do the ballooning?

I will dive in this in the following days, thanks for the work Vlastimil


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