Commenting on the series "Reduce writeback from page reclaim context V6" Andrew Morton noted; direct-reclaim wants to write a dirty page because that page is in the zone which the caller wants to allocate from! Telling the flusher threads to perform generic writeback will sometimes cause them to just gum the disk up with pages from different zones, making it even harder/slower to allocate a page from the zones we're interested in, no? On the machines used to test the series, there were relatively few zones and only one BDI so the scenario describes is a possibility. This series is a very early prototype series aimed at mitigating the problem. Patch 1 adds wakeup_flusher_threads_pages() which takes a list of pages from page reclaim. Each inode belonging to a page on the list is marked I_DIRTY_RECLAIM. When the flusher thread wakes, inodes with this tag are unconditionally moved to the wb->b_io list for writing. Patch 2 notes that writing back inodes does not necessarily write back pages belonging to the zone page reclaim is concerned with. In response, it adds a zone and counter to wb_writeback_work. As pages from the target zone are written, the zone-specific counter is updated. When the flusher thread then checks the zone counters if a specific zone is being targeted. While more pages may be written than necessary, the assumption is that the pages need cleaning eventually, the inode must be relatively old to have pages at the end of the LRU, the IO will be relatively efficient due to less random seeks and that pages from the target zone will still be cleaned. Testing did not show any significant differences in terms of reducing dirty file pages being written back but the lack of multiple BDIs and NUMA nodes in the test rig is a problem. Maybe someone else has access to a more suitable test rig. Any comment as to the suitability for such a direction? fs/fs-writeback.c | 83 +++++++++++++++++++++++++++++++++++++++++--- include/linux/fs.h | 5 ++- include/linux/writeback.h | 5 +++ mm/page-writeback.c | 12 ++++++- mm/vmscan.c | 11 ++++-- 5 files changed, 103 insertions(+), 13 deletions(-) -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
