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| -rw-r--r-- | Documentation/cgroups/blkio-controller.txt | 83 |
1 files changed, 78 insertions, 5 deletions
diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt index cd556b914786..68b6a6a470b0 100644 --- a/Documentation/cgroups/blkio-controller.txt +++ b/Documentation/cgroups/blkio-controller.txt @@ -387,8 +387,81 @@ groups and put applications in that group which are not driving enough IO to keep disk busy. In that case set group_idle=0, and CFQ will not idle on individual groups and throughput should improve. -What works -========== -- Currently only sync IO queues are support. All the buffered writes are - still system wide and not per group. Hence we will not see service - differentiation between buffered writes between groups. +Writeback +========= + +Page cache is dirtied through buffered writes and shared mmaps and +written asynchronously to the backing filesystem by the writeback +mechanism. Writeback sits between the memory and IO domains and +regulates the proportion of dirty memory by balancing dirtying and +write IOs. + +On traditional cgroup hierarchies, relationships between different +controllers cannot be established making it impossible for writeback +to operate accounting for cgroup resource restrictions and all +writeback IOs are attributed to the root cgroup. + +If both the blkio and memory controllers are used on the v2 hierarchy +and the filesystem supports cgroup writeback, writeback operations +correctly follow the resource restrictions imposed by both memory and +blkio controllers. + +Writeback examines both system-wide and per-cgroup dirty memory status +and enforces the more restrictive of the two. Also, writeback control +parameters which are absolute values - vm.dirty_bytes and +vm.dirty_background_bytes - are distributed across cgroups according +to their current writeback bandwidth. + +There's a peculiarity stemming from the discrepancy in ownership +granularity between memory controller and writeback. While memory +controller tracks ownership per page, writeback operates on inode +basis. cgroup writeback bridges the gap by tracking ownership by +inode but migrating ownership if too many foreign pages, pages which +don't match the current inode ownership, have been encountered while +writing back the inode. + +This is a conscious design choice as writeback operations are +inherently tied to inodes making strictly following page ownership +complicated and inefficient. The only use case which suffers from +this compromise is multiple cgroups concurrently dirtying disjoint +regions of the same inode, which is an unlikely use case and decided +to be unsupported. Note that as memory controller assigns page +ownership on the first use and doesn't update it until the page is +released, even if cgroup writeback strictly follows page ownership, +multiple cgroups dirtying overlapping areas wouldn't work as expected. +In general, write-sharing an inode across multiple cgroups is not well +supported. + +Filesystem support for cgroup writeback +--------------------------------------- + +A filesystem can make writeback IOs cgroup-aware by updating +address_space_operations->writepage[s]() to annotate bio's using the +following two functions. + +* wbc_init_bio(@wbc, @bio) + + Should be called for each bio carrying writeback data and associates + the bio with the inode's owner cgroup. Can be called anytime + between bio allocation and submission. + +* wbc_account_io(@wbc, @page, @bytes) + + Should be called for each data segment being written out. While + this function doesn't care exactly when it's called during the + writeback session, it's the easiest and most natural to call it as + data segments are added to a bio. + +With writeback bio's annotated, cgroup support can be enabled per +super_block by setting MS_CGROUPWB in ->s_flags. This allows for +selective disabling of cgroup writeback support which is helpful when +certain filesystem features, e.g. journaled data mode, are +incompatible. + +wbc_init_bio() binds the specified bio to its cgroup. Depending on +the configuration, the bio may be executed at a lower priority and if +the writeback session is holding shared resources, e.g. a journal +entry, may lead to priority inversion. There is no one easy solution +for the problem. Filesystems can try to work around specific problem +cases by skipping wbc_init_bio() or using bio_associate_blkcg() +directly. |