Include scatter_chunk_waste in arc_size

The ARC caches data in scatter ABD's, which are collections of pages,
which are typically 4K.  Therefore, the space used to cache each block
is rounded up to a multiple of 4K.  The ABD subsystem tracks this wasted
memory in the `scatter_chunk_waste` kstat.  However, the ARC's `size` is
not aware of the memory used by this round-up, it only accounts for the
size that it requested from the ABD subsystem.

Therefore, the ARC is effectively using more memory than it is aware of,
due to the `scatter_chunk_waste`.  This impacts observability, e.g.
`arcstat` will show that the ARC is using less memory than it
effectively is.  It also impacts how the ARC responds to memory
pressure.  As the amount of `scatter_chunk_waste` changes, it appears to
the ARC as memory pressure, so it needs to resize `arc_c`.

If the sector size (`1<<ashift`) is the same as the page size (or
larger), there won't be any waste.  If the (compressed) block size is
relatively large compared to the page size, the amount of
`scatter_chunk_waste` will be small, so the problematic effects are
minimal.

However, if using 512B sectors (`ashift=9`), and the (compressed) block
size is small (e.g. `compression=on` with the default `volblocksize=8k`
or a decreased `recordsize`), the amount of `scatter_chunk_waste` can be
very large.  On a production system, with `arc_size` at a constant 50%
of memory, `scatter_chunk_waste` has been been observed to be 10-30% of
memory.

This commit adds `scatter_chunk_waste` to `arc_size`, and adds a new
`waste` field to `arcstat`.  As a result, the ARC's memory usage is more
observable, and `arc_c` does not need to be adjusted as frequently.

Reviewed-by: Pavel Zakharov <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: George Wilson <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Signed-off-by: Matthew Ahrens <[email protected]>
Closes #10701 

1 files changed, 5 insertions, 4 deletions

diff --git a/module/os/freebsd/zfs/abd_os.c b/module/os/freebsd/zfs/abd_os.c
index 6fb43d6bb..a7bda509b 100644
--- a/module/os/freebsd/zfs/abd_os.c
+++ b/module/os/freebsd/zfs/abd_os.c
@@ -131,16 +131,17 @@ abd_update_scatter_stats(abd_t *abd, abd_stats_op_t op)
 {
 	size_t n = abd_scatter_chunkcnt(abd);
 	ASSERT(op == ABDSTAT_INCR || op == ABDSTAT_DECR);
+	int waste = n * zfs_abd_chunk_size - abd->abd_size;
 	if (op == ABDSTAT_INCR) {
 		ABDSTAT_BUMP(abdstat_scatter_cnt);
 		ABDSTAT_INCR(abdstat_scatter_data_size, abd->abd_size);
-		ABDSTAT_INCR(abdstat_scatter_chunk_waste,
-		    n * zfs_abd_chunk_size - abd->abd_size);
+		ABDSTAT_INCR(abdstat_scatter_chunk_waste, waste);
+		arc_space_consume(waste, ARC_SPACE_ABD_CHUNK_WASTE);
 	} else {
 		ABDSTAT_BUMPDOWN(abdstat_scatter_cnt);
 		ABDSTAT_INCR(abdstat_scatter_data_size, -(int)abd->abd_size);
-		ABDSTAT_INCR(abdstat_scatter_chunk_waste,
-		    abd->abd_size - n * zfs_abd_chunk_size);
+		ABDSTAT_INCR(abdstat_scatter_chunk_waste, -waste);
+		arc_space_return(waste, ARC_SPACE_ABD_CHUNK_WASTE);
 	}
 }