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SubjectRe: [RFC 0/5] disable pcplists during page isolation
On 09.09.20 13:27, wrote:
> 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?

No, they only add more memory blocks and online them. Memory hot(un)plug
is an expensive operation already, I don't think that's an issue.


David / dhildenb

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