linux-kernel - mmap() scalability in the presence of the MAP

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Message-ID: <1357145418.5429.17.camel@mesosphere.localdomain>
Date:	Wed, 02 Jan 2013 23:50:18 +0700
From:	Roman Dubtsov <dubtsov@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: mmap() scalability in the presence of the MAP_POPULATE flag

Concurrent mmap() calls from the same process are serialized via downing
mm->mmap_sem for write. This means that operations like populating the
pages which do not alter vmas are also performed serially. Anecdotal
data from two machines I have access to is that populating pages by
touching them in a loop outside of mmap() improves performance of the
synthetic micro-benchmark by ~40% in the worst case. A crude patch that
modifies vm_mmap_pgoff() to call make_pages_present() outside of
do_mmap_pgoff() after upping the semaphore when MAP_POPULATE is present
brings identical performance improvement.

Is there an interest in fixing this or concurrent mmaps() from the same
process are too much of a corner case to worry about it?

Regards,
Roma 

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