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 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
