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

[Eric B Munson]

On Mon, 11 May 2015, Eric B Munson wrote:

> 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
> 

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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