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

[David Hildenbrand]

On 09.09.20 13:27, osalvador@xxxxxxx 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.

-- 
Thanks,

David / dhildenb