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

[Michal Hocko]

On Wed 13-05-15 11:00:36, Eric B Munson wrote:
> 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

Have you played with batching? Has it helped? Anyway it is to be
expected that the overhead will be higher than a single mmap call. The
question is whether you can live with it because adding a new semantic
to mlock sounds trickier and MAP_LOCKED is tricky enough already...

-- 
Michal Hocko
SUSE Labs
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