writeback: skip balance_dirty_pages() for in-memory fs
This avoids unnecessary checks and dirty throttling on tmpfs/ramfs. Notes about the tmpfs/ramfs behavior changes: As for 2.6.36 and older kernels, the tmpfs writes will sleep inside balance_dirty_pages() as long as we are over the (dirty+background)/2 global throttle threshold. This is because both the dirty pages and threshold will be 0 for tmpfs/ramfs. Hence this test will always evaluate to TRUE: dirty_exceeded = (bdi_nr_reclaimable + bdi_nr_writeback >= bdi_thresh) || (nr_reclaimable + nr_writeback >= dirty_thresh); For 2.6.37, someone complained that the current logic does not allow the users to set vm.dirty_ratio=0. So commit 4cbec4c8 changed the test to dirty_exceeded = (bdi_nr_reclaimable + bdi_nr_writeback > bdi_thresh) || (nr_reclaimable + nr_writeback > dirty_thresh); So 2.6.37 will behave differently for tmpfs/ramfs: it will never get throttled unless the global dirty threshold is exceeded (which is very unlikely to happen; once happen, will block many tasks). I'd say that the 2.6.36 behavior is very bad for tmpfs/ramfs. It means for a busy writing server, tmpfs write()s may get livelocked! The "inadvertent" throttling can hardly bring help to any workload because of its "either no throttling, or get throttled to death" property. So based on 2.6.37, this patch won't bring more noticeable changes. CC: Hugh Dickins <hughd@google.com> Acked-by: NRik van Riel <riel@redhat.com> Acked-by: NPeter Zijlstra <a.p.zijlstra@chello.nl> Reviewed-by: NMinchan Kim <minchan.kim@gmail.com> Signed-off-by: NWu Fengguang <fengguang.wu@intel.com>
Showing
想要评论请 注册 或 登录