Date: Wed, 13 May 2015 11:00:36 -0400
From: Eric B Munson <emunson@akamai.com>
To: Michal Hocko <mhocko@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Shuah Khan <shuahkh@osg.samsung.com>, linux-alpha@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mips@linux-mips.org,
        linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        sparclinux@vger.kernel.org, linux-xtensa@linux-xtensa.org,
        linux-mm@kvack.org, linux-arch@vger.kernel.org,
        linux-api@vger.kernel.org
Subject: Re: [PATCH 0/3] Allow user to request memory to be locked on page
 fault
Message-ID: <20150513150036.GG1227@akamai.com>
References: <1431113626-19153-1-git-send-email-emunson@akamai.com>
 <20150508124203.6679b1d35ad9555425003929@linux-foundation.org>
 <20150511180631.GA1227@akamai.com>
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On Mon, 11 May 2015, Eric B Munson wrote:

> On Fri, 08 May 2015, Andrew Morton wrote:
>=20
> > On Fri,  8 May 2015 15:33:43 -0400 Eric B Munson <emunson@akamai.com> w=
rote:
> >=20
> > > mlock() allows a user to control page out of program memory, but this
> > > comes at the cost of faulting in the entire mapping when it is
> > > allocated.  For large mappings where the entire area is not necessary
> > > this is not ideal.
> > >=20
> > > This series introduces new flags for mmap() and mlockall() that allow=
 a
> > > user to specify that the covered are should not be paged out, but only
> > > after the memory has been used the first time.
> >=20
> > Please tell us much much more about the value of these changes: the use
> > cases, the behavioural improvements and performance results which the
> > patchset brings to those use cases, etc.
> >=20
>=20
> To illustrate the proposed use case I wrote a quick program that mmaps
> a 5GB file which is filled with random data and accesses 150,000 pages
> from that mapping.  Setup and processing were timed separately to
> illustrate the differences between the three tested approaches.  the
> setup portion is simply the call to mmap, the processing is the
> accessing of the various locations in  that mapping.  The following
> values are in milliseconds and are the averages of 20 runs each with a
> call to echo 3 > /proc/sys/vm/drop_caches between each run.
>=20
> The first mapping was made with MAP_PRIVATE | MAP_LOCKED as a baseline:
> Startup average:    9476.506
> Processing average: 3.573
>=20
> The second mapping was simply MAP_PRIVATE but each page was passed to
> mlock() before being read:
> Startup average:    0.051
> Processing average: 721.859
>=20
> The final mapping was MAP_PRIVATE | MAP_LOCKONFAULT:
> Startup average:    0.084
> Processing average: 42.125
>=20

Michal's suggestion of changing protections and locking in a signal
handler was better than the locking as needed, but still significantly
more work required than the LOCKONFAULT case.

Startup average:    0.047
Processing average: 86.431


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