Re: [PATCH 0/3] Allow user to request memory to be locked on page fault

From: Eric B Munson
Date: Mon May 11 2015 - 14:06:41 EST

On Fri, 08 May 2015, Andrew Morton wrote:

> On Fri, 8 May 2015 15:33:43 -0400 Eric B Munson <emunson@xxxxxxxxxx> wrote:
> > 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.
> >
> > 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.
> 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.

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.

The first mapping was made with MAP_PRIVATE | MAP_LOCKED as a baseline:
Startup average: 9476.506
Processing average: 3.573

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

The final mapping was MAP_PRIVATE | MAP_LOCKONFAULT:
Startup average: 0.084
Processing average: 42.125

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