On Thu, 14 May 2015, Michal Hocko wrote: > 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... > I reworked the experiment to better cover the batching solution. The same 5GB data file is used, however instead of 150,000 accesses at regular intervals, the test program now does 15,000,000 accesses to random pages in the mapping. The rest of the setup remains the same. mmap with MAP_LOCKED: Setup avg: 11821.193 Processing avg: 3404.286 mmap with mlock() before each access: Setup avg: 0.054 Processing avg: 34263.201 mmap with PROT_NONE and signal handler and batch size of 1 page: With the default value in max_map_count, this gets ENOMEM as I attempt to change the permissions, after upping the sysctl significantly I get: Setup avg: 0.050 Processing avg: 67690.625 mmap with PROT_NONE and signal handler and batch size of 8 pages: Setup avg: 0.098 Processing avg: 37344.197 mmap with PROT_NONE and signal handler and batch size of 16 pages: Setup avg: 0.0548 Processing avg: 29295.669 mmap with MAP_LOCKONFAULT: Setup avg: 0.073 Processing avg: 18392.136 The signal handler in the batch cases faulted in memory in two steps to avoid having to know the start and end of the faulting mapping. The first step covers the page that caused the fault as we know that it will be possible to lock. The second step speculatively tries to mlock and mprotect the batch size - 1 pages that follow. There may be a clever way to avoid this without having the program track each mapping to be covered by this handeler in a globally accessible structure, but I could not find it. These results show that if the developer knows that a majority of the mapping will be used, it is better to try and fault it in at once, otherwise MAP_LOCKONFAULT is significantly faster. Eric
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