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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