From 1a12d8bd7b2998be01ee55edb64e7473728abb9c Mon Sep 17 00:00:00 2001 From: Wu Fengguang Date: Sun, 29 Aug 2010 13:28:09 -0600 Subject: writeback: scale IO chunk size up to half device bandwidth Originally, MAX_WRITEBACK_PAGES was hard-coded to 1024 because of a concern of not holding I_SYNC for too long. (At least, that was the comment previously.) This doesn't make sense now because the only time we wait for I_SYNC is if we are calling sync or fsync, and in that case we need to write out all of the data anyway. Previously there may have been other code paths that waited on I_SYNC, but not any more. -- Theodore Ts'o So remove the MAX_WRITEBACK_PAGES constraint. The writeback pages will adapt to as large as the storage device can write within 500ms. XFS is observed to do IO completions in a batch, and the batch size is equal to the write chunk size. To avoid dirty pages to suddenly drop out of balance_dirty_pages()'s dirty control scope and create large fluctuations, the chunk size is also limited to half the control scope. The balance_dirty_pages() control scrope is [(background_thresh + dirty_thresh) / 2, dirty_thresh] which is by default [15%, 20%] of global dirty pages, whose range size is dirty_thresh / DIRTY_FULL_SCOPE. The adpative write chunk size will be rounded to the nearest 4MB boundary. http://bugzilla.kernel.org/show_bug.cgi?id=13930 CC: Theodore Ts'o CC: Dave Chinner CC: Chris Mason CC: Peter Zijlstra Signed-off-by: Wu Fengguang diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index 227ff12..50445cf 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -30,15 +30,6 @@ #include "internal.h" /* - * The maximum number of pages to writeout in a single bdi flush/kupdate - * operation. We do this so we don't hold I_SYNC against an inode for - * enormous amounts of time, which would block a userspace task which has - * been forced to throttle against that inode. Also, the code reevaluates - * the dirty each time it has written this many pages. - */ -#define MAX_WRITEBACK_PAGES 1024L - -/* * Passed into wb_writeback(), essentially a subset of writeback_control */ struct wb_writeback_work { @@ -515,7 +506,8 @@ static bool pin_sb_for_writeback(struct super_block *sb) return false; } -static long writeback_chunk_size(struct wb_writeback_work *work) +static long writeback_chunk_size(struct backing_dev_info *bdi, + struct wb_writeback_work *work) { long pages; @@ -534,8 +526,13 @@ static long writeback_chunk_size(struct wb_writeback_work *work) */ if (work->sync_mode == WB_SYNC_ALL || work->tagged_writepages) pages = LONG_MAX; - else - pages = min(MAX_WRITEBACK_PAGES, work->nr_pages); + else { + pages = min(bdi->avg_write_bandwidth / 2, + global_dirty_limit / DIRTY_SCOPE); + pages = min(pages, work->nr_pages); + pages = round_down(pages + MIN_WRITEBACK_PAGES, + MIN_WRITEBACK_PAGES); + } return pages; } @@ -600,7 +597,7 @@ static long writeback_sb_inodes(struct super_block *sb, continue; } __iget(inode); - write_chunk = writeback_chunk_size(work); + write_chunk = writeback_chunk_size(wb->bdi, work); wbc.nr_to_write = write_chunk; wbc.pages_skipped = 0; diff --git a/include/linux/writeback.h b/include/linux/writeback.h index b625073..f1bfa12e 100644 --- a/include/linux/writeback.h +++ b/include/linux/writeback.h @@ -8,6 +8,10 @@ #include /* + * The 1/4 region under the global dirty thresh is for smooth dirty throttling: + * + * (thresh - thresh/DIRTY_FULL_SCOPE, thresh) + * * The 1/16 region above the global dirty limit will be put to maximum pauses: * * (limit, limit + limit/DIRTY_MAXPAUSE_AREA) @@ -25,9 +29,16 @@ * knocks down the global dirty threshold quickly, in which case the global * dirty limit will follow down slowly to prevent livelocking all dirtier tasks. */ +#define DIRTY_SCOPE 8 +#define DIRTY_FULL_SCOPE (DIRTY_SCOPE / 2) #define DIRTY_MAXPAUSE_AREA 16 #define DIRTY_PASSGOOD_AREA 8 +/* + * 4MB minimal write chunk size + */ +#define MIN_WRITEBACK_PAGES (4096UL >> (PAGE_CACHE_SHIFT - 10)) + struct backing_dev_info; /* -- cgit v0.10.2