On Mon, 9 Sep 2019, Mike Snitzer wrote: > On Fri, Sep 06 2019 at 3:45am -0400, > Martijn Coenen <maco@xxxxxxxxxxx> wrote: > > > The upper limit on the cache size of a client is currently determined by > > dividing the total cache size by the number of clients. However, in some > > cases it is beneficial to give one client a higher limit than others; an > > example is a device with many dm-verity targets, where one target has a > > very large hashtree, and all the others have a small hashtree. Giving > > the target with the large hashtree a higher limit will be beneficial. > > Another example is dm-verity-fec: FEC is only used in (rare) error > > conditions, yet for every dm-verity target with FEC, we create two FEC > > dm-bufio clients, which together have a higher cache limit than the > > dm-verity target itself. > > > > This patchset allows a client to indicate a maximum cache size for its > > client; if that maximum is lower than the calculated per-client limit, > > that maximum will be used instead, and the freed up cache size will be > > allocated to other clients (that haven't set a maximum). > > > > Note that this algorithm is not perfect; if we have 100MB with 3 > > clients, where the first set a max of 1MB, the second set a max of 40MB, > > and the third set no maximumm, the ideal allocation would be 1:40:59, > > respectively. However, because the initial per-client limit is 100 / 3 > > =~33MB, the requested max of 40MB is over the per-client limit, and > > instead the allocation will end up being ~ 1:40:49. This is still better > > than the original 33:33:33 allocation. An iterative algorithm could do > > better, but it also complicates the code significantly. > > Definitely not very intuitive.. but yes I think it is a reasonable > tradeoff between your goals and further code complexity to be able to > achieve the "ideal". > > Think the documented example can be made clearer by documenting that > dm_bufio_cache_size_per_client = 49. And that _that_ is the reason why > the client that didn't set a maximum is bounded to 49. > > Overall I think this patch looks reasonable, but I'd like Mikulas to > review this closer before I pick it up. > > Thanks, > Mike I think the proper way how to solve this problem with large amount of clients is to have a global queue holding all the buffers and clean up the oldest buffers in the queue. So that if one instance of the dm-verity target uses the buffer cache heavily, it can evict buffers loaded by other inactive dm-verity instances. I am now working on the global queue patch. Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
