linux-kernel - Re: [PATCHv6 00/22] Transparent huge page cache: phase 1, everything but mmap()

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Message-ID: <52499875.4060101@sr71.net>
Date:	Mon, 30 Sep 2013 08:27:49 -0700
From:	Dave Hansen <dave@...1.net>
To:	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	linux-mm@...ck.org, Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hillf Danton <dhillf@...il.com>, Ning Qu <quning@...gle.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv6 00/22] Transparent huge page cache: phase 1, everything
 but mmap()

On 09/30/2013 03:02 AM, Mel Gorman wrote:
> I am afraid I never looked too closely once I learned that the primary
> motivation for this was relieving iTLB pressure in a very specific
> case. AFAIK, this is not a problem in the vast majority of modern CPUs
> and I found it very hard to be motivated to review the series as a result.
> I suspected that in many cases that the cost of IO would continue to dominate
> performance instead of TLB pressure. I also found it unlikely that there
> was a workload that was tmpfs based that used enough memory to be hurt
> by TLB pressure. My feedback was that a much more compelling case for the
> series was needed but this discussion all happened on IRC unfortunately.

FWIW, I'm mostly intrigued by the possibilities of how this can speed up
_software_, and I'm rather uninterested in what it can do for the TLB.
Page cache is particularly painful today, precisely because hugetlbfs
and anonymous-thp aren't available there.  If you have an app with
hundreds of GB of files that it wants to mmap(), even if it's in the
page cache, it takes _minutes_ to just fault in.  One example:

	https://lkml.org/lkml/2013/6/27/698
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