From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <owner-linux-mm@kvack.org>
Subject: Re: [PATCH 0/7 v1] Speed up page cache truncation
To: Jan Kara <jack@suse.cz>, Andi Kleen <ak@linux.intel.com>
References: <20171010151937.26984-1-jack@suse.cz>
 <878tgisyo6.fsf@linux.intel.com> <20171011080658.GK3667@quack2.suse.cz>
Cc: linux-mm@kvack.org, Mel Gorman <mgorman@suse.de>,
 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
 linux-fsdevel@vger.kernel.org
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <e596a6d7-4858-8fe6-c315-8a285748a31a@intel.com>
Date: Wed, 11 Oct 2017 10:34:47 -0700
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On 10/11/2017 01:06 AM, Jan Kara wrote:
>>> when rebasing our enterprise distro to a newer kernel (from 4.4 to 4.12) we
>>> have noticed a regression in bonnie++ benchmark when deleting files.
>>> Eventually we have tracked this down to a fact that page cache truncation got
>>> slower by about 10%. There were both gains and losses in the above interval of
>>> kernels but we have been able to identify that commit 83929372f629 "filemap:
>>> prepare find and delete operations for huge pages" caused about 10% regression
>>> on its own.
>> It's odd that just checking if some pages are huge should be that
>> expensive, but ok ..
> Yeah, I was surprised as well but profiles were pretty clear on this - part
> of the slowdown was caused by loads of page->_compound_head (PageTail()
> and page_compound() use that) which we previously didn't have to load at
> all, part was in hpage_nr_pages() function and its use.

Well, page->_compound_head is part of the same cacheline as the rest of
the page, and the page is surely getting touched during truncation at
_some_ point.  The hpage_nr_pages() might cause the cacheline to get
loaded earlier than before, but I can't imagine that it's that expensive.

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