On Fri, 15 May 2015, Eric B Munson wrote: > 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 Is there anything else I can add to the discussion here?
Attachment:
signature.asc
Description: Digital signature
