Re: [PATCH 0/6] Enable parallel page migration

[Mel Gorman]

On Wed, Mar 08, 2017 at 09:34:27PM +0530, Anshuman Khandual wrote:
> > Any comments, suggestions are welcome.
> 
> Hello Vlastimil/Michal/Minchan/Mel/Dave,
> 
> Apart from the comments from Naoya on a different thread posted by Zi
> Yan, I did not get any more review comments on this series. Could you
> please kindly have a look on the over all design and its benefits from
> page migration performance point of view and let me know your views.
> Thank you.
> 

I didn't look into the patches in detail except to get a general feel
for how it works and I'm not convinced that it's a good idea at all.

I accept that memory bandwidth utilisation may be higher as a result but
consider the impact. THP migrations are relatively rare and when they
occur, it's in the context of a single thread. To parallelise the copy,
an allocation, kmap and workqueue invocation are required. There may be a
long delay before the workqueue item can start which may exceed the time
to do a single copy if the CPUs on a node are saturated. Furthermore, a
single thread can preempt operations of other unrelated threads and incur
CPU cache pollution and future misses on unrelated CPUs. It's compounded by
the fact that a high priority system workqueue is used to do the operation,
one that is used for CPU hotplug operations and rolling back when a netdevice
fails to be registered. It treats a hugepage copy as an essential operation
that can preempt all other work which is very questionable.

The series leader has no details on a workload that is bottlenecked by
THP migrations and even if it is, the primary question should be *why*
THP migrations are so frequent and alleviating that instead of
preempting multiple CPUs to do the work.

-- 
Mel Gorman
SUSE Labs