[PATCH 5/5] xfs: reduce buffer log item shadow allocations

Dave Chinner
2021-01-28T04:42:42Z

From: Dave Chinner <dchinner@redhat.com>

When we modify btrees repeatedly, we regularly increase the size of
the logged region by a single chunk at a time (per transaction
commit). This results in the CIL formatting code having to
reallocate the log vector buffer every time the buffer dirty region
grows. Hence over a typical 4kB btree buffer, we might grow the log
vector 4096/128 = 32x over a short period where we repeatedly add
or remove records to/from the buffer over a series of running
transaction. This means we are doing 32 memory allocations and frees
over this time during a performance critical path in the journal.

The amount of space tracked in the CIL for the object is calculated
during the ->iop_format() call for the buffer log item, but the
buffer memory allocated for it is calculated by the ->iop_size()
call. The size callout determines the size of the buffer, the format
call determines the space used in the buffer.

Hence we can oversize the buffer space required in the size
calculation without impacting the amount of space used and accounted
to the CIL for the changes being logged. This allows us to reduce
the number of allocations by rounding up the buffer size to allow
for future growth. This can safe a substantial amount of CPU time in
this path:

- 46.52% 2.02% [kernel] [k] xfs_log_commit_cil
- 44.49% xfs_log_commit_cil
- 30.78% _raw_spin_lock
- 30.75% do_raw_spin_lock
30.27% __pv_queued_spin_lock_slowpath

(oh, ouch!)
....
- 1.05% kmem_alloc_large
- 1.02% kmem_alloc
0.94% __kmalloc

This overhead here us what this patch is aimed at. After:

- 0.76% kmem_alloc_large ▒
- 0.75% kmem_alloc ▒
0.70% __kmalloc ▒

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
fs/xfs/xfs_buf_item.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c
index 17960b1ce5ef..0628a65d9c55 100644
--- a/fs/xfs/xfs_buf_item.c
+++ b/fs/xfs/xfs_buf_item.c
@@ -142,6 +142,7 @@ xfs_buf_item_size(
{
struct xfs_buf_log_item *bip = BUF_ITEM(lip);
int i;
+ int bytes;

ASSERT(atomic_read(&bip->bli_refcount) > 0);
if (bip->bli_flags & XFS_BLI_STALE) {
@@ -173,7 +174,7 @@ xfs_buf_item_size(
}

/*
- * the vector count is based on the number of buffer vectors we have
+ * The vector count is based on the number of buffer vectors we have
* dirty bits in. This will only be greater than one when we have a
* compound buffer with more than one segment dirty. Hence for compound
* buffers we need to track which segment the dirty bits correspond to,
@@ -181,10 +182,18 @@ xfs_buf_item_size(
* count for the extra buf log format structure that will need to be
* written.
*/
+ bytes = 0;
for (i = 0; i < bip->bli_format_count; i++) {
xfs_buf_item_size_segment(bip, &bip->bli_formats[i],
- nvecs, nbytes);
+ nvecs, &bytes);
}
+
+ /*
+ * Round up the buffer size required to minimise the number of memory
+ * allocations that need to be done as this item grows when relogged by
+ * repeated modifications.
+ */
+ *nbytes = round_up(bytes, 512);
trace_xfs_buf_item_size(bip);
}

--
2.28.0