On Mon, Mar 29, 2010 at 10:29:37AM -0500, Eric Sandeen wrote: > This patch makes mpage_add_bh_to_extent stop the loop after we've > accumulated 2048 pages, by setting mpd->io_done = 1; which ultimately > causes the write_cache_pages loop to break. > > Repeating the test with a dirty_ratio of 80 (to leave something for > fsync to do), I don't see huge IO performance gains, but the reduction > in cpu usage is striking: 80% usage with stock, and 2% with the > below patch. Instrumenting the loop in write_cache_pages clearly > shows that we are wasting time here. > > It'd be better to not have a magic number of 2048 in here, so I'll > look for a cleaner way to get this info out of mballoc; I still need > to look at what Aneesh has in the patch queue, that might help. > This is something we could probably put in for now, though; the 2048 > is already enshrined in a comment in inode.c, at least. I wonder if a better way of fixing this is to changing mpage_da_map_pages() to call ext4_get_blocks() multiple times. This should be a lot easier after we integrate mpage_da_submit_io() into mpage_da_map_pages(). That way we can way more efficient; in a loop, we accumulate the pages, call ext4_get_blocks(), then submit the IO (as a single block I/O submission, instead of 4k at a time through ext4_writepages()), and then call ext4_get_blocks() again, etc. I'm willing to include this patch as an interim stopgap, but eventually, I think we need to refactor and reorganize mpage_da_map_pages() and and mpage_da_submit_IO(), and let them call mballoc (via ext4_get_blocks) multiple times in a loop. Thoughts, suggestions? - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
