On Wed, May 19, 2021 at 09:08:54PM +0200, Christoph Hellwig wrote:
> We did not find any page, we're allocating them all from the page
> allocator.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
> fs/xfs/xfs_buf.c | 2 --
> 1 file changed, 2 deletions(-)
>
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 9c64c374411081..76240d84d58b61 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -436,8 +436,6 @@ xfs_buf_alloc_pages(
> goto retry;
> }
>
> - XFS_STATS_INC(bp->b_mount, xb_page_found);
> -
> nbytes = min_t(size_t, size, PAGE_SIZE);
> size -= nbytes;
> bp->b_pages[i] = page;
NACK. This is actually telling us that a page was allocated
successfully. I just used this very stat in combination with the
page allocate failure stat (xb_page_retries) to determine that the
bulk alloc code was failing to allocate all the pages asked for at
least 20% of the time.
$ xfs_stats.pl
.....
xs_ig_dup............ 0 Buf Statistics
xs_ig_reclaims....... 38377759 pb_get................ 432887411
xs_ig_attrchg........ 1 pb_create............. 1839653
Log Operations pb_get_locked......... 431047794
xs_log_writes........ 71572 pb_get_locked_waited.. 346
xs_log_blocks........ 36644864 pb_busy_locked........ 13615
xs_log_noiclogs...... 265 pb_miss_locked........ 1839651
xs_log_force......... 521 pb_page_retries....... 488537
xs_log_force_sleep... 495 pb_page_found......... 1839431
pb_get_read........... 577
See the pb_miss_locked, pb_page_found and pb_page_retries numbers?
Almost all cache misses required page (rather than heap) allocation,
and 25% of them bulk allocation failed to allocate all pages in a
single call.
So, yeah, the buffer cache stats are useful diagnostic information
that I use a lot...
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
