linux-kernel - Re: [PATCH v3] Linux VM workaround for Knights Landing A/D leak

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Message-ID: <57630ADE.3040900@linux.intel.com>
Date:	Thu, 16 Jun 2016 13:23:58 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Lukasz Anaczkowski <lukasz.anaczkowski@...el.com>, hpa@...or.com,
	mingo@...hat.com, tglx@...utronix.de, ak@...ux.intel.com,
	kirill.shutemov@...ux.intel.com, mhocko@...e.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Cc:	harish.srinivasappa@...el.com, lukasz.odzioba@...el.com,
	grzegorz.andrejczuk@...el.com, lukasz.daniluk@...el.com
Subject: Re: [PATCH v3] Linux VM workaround for Knights Landing A/D leak

On 06/16/2016 08:14 AM, Lukasz Anaczkowski wrote:
> For reclaim this brings the performance back to before Mel's
> flushing changes, but for unmap it disables batching.

This turns out to be pretty catastrophic for unmap.  In a workload that
uses, say 200 hardware threads and alloc/frees() a few MB/sec, this ends
up costing hundreds of thousands of extra received IPIs.  10MB=~2500
ptes, and at with 200 threads, that's 250,000 IPIs received just to free
10MB of memory.

The initial testing we did on this was on a *bunch* of threads all doing
alloc/free.  But this is bottlenecked on other things, like mmap_sem
being held for write.

The scenario that we really needed to test here was on lots of threads
doing processing and 1 thread doing alloc/free.