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

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Date:   Thu, 9 Mar 2017 15:09:04 +0000
From:   Mel Gorman <mgorman@...e.de>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhocko@...e.com,
        vbabka@...e.cz, minchan@...nel.org,
        aneesh.kumar@...ux.vnet.ibm.com, bsingharora@...il.com,
        srikar@...ux.vnet.ibm.com, haren@...ux.vnet.ibm.com,
        jglisse@...hat.com, dave.hansen@...el.com,
        dan.j.williams@...el.com, zi.yan@...rutgers.edu,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Subject: Re: [PATCH 0/6] Enable parallel page migration

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