On Thu, Mar 03, 2011 at 02:45:11PM +0800, Wu Fengguang wrote: > Call balance_dirty_pages_ratelimit_nr() on every 32 pages dirtied. > > Tests show that original larger intervals can easily make the bdi > dirty limit exceeded on 100 concurrent dd. > > CC: Chris Mason <chris.mason@xxxxxxxxxx> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> > --- > fs/btrfs/file.c | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) > > --- linux-next.orig/fs/btrfs/file.c 2011-03-02 20:15:19.000000000 +0800 > +++ linux-next/fs/btrfs/file.c 2011-03-02 20:35:07.000000000 +0800 > @@ -949,9 +949,8 @@ static ssize_t btrfs_file_aio_write(stru > } > > iov_iter_init(&i, iov, nr_segs, count, num_written); > - nrptrs = min((iov_iter_count(&i) + PAGE_CACHE_SIZE - 1) / > - PAGE_CACHE_SIZE, PAGE_CACHE_SIZE / > - (sizeof(struct page *))); > + nrptrs = min(DIV_ROUND_UP(iov_iter_count(&i), PAGE_CACHE_SIZE), > + min(32UL, PAGE_CACHE_SIZE / sizeof(struct page *))); You're basically hardcoding the maximum to 32 pages here, because PAGE_CACHE_SIZE / sizeof(page *) is always going to be much larger than 32. This means that you are effectively neutering the large write efficiencies of btrfs - you're reducing the delayed allocation sizes from 512 * PAGE_CACHE_SIZE down to 32 * PAGE_CACHE_SIZE. This will increase the overhead of the write process for btrfs for large IOs. Also, I've got some multipage write modifications that allow 1024 pages at a time between mapping/allocation calls with XFS - once again for improving the efficiencies of the extent mapping/allocations in the write path. If the new writeback throttling algorithms don't work with large numbers of pages being copied in a single go, then that's a problem. As it is, if 100 concurrent dd's can overrun the dirty limit w/ 512 pages at a time, then 1000 concurrent dd's w/ 32 pages at a time is just as likely to overrun it, too. We support 4096 CPU systems, so a few thousand concurrent writers is not out of the question. Hence I don't think just reducing the number of pages between dirty balance calls is a sufficient solution.... Cheers, Dave.. -- Dave Chinner david@xxxxxxxxxxxxx -- 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
