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
