Re: [RFC 0/5] disable pcplists during page isolation

[osalvador@suse.de]

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