On 16/06/2025 07:28, Damien Le Moal wrote:
For a device that does not advertize an optimal I/O size, the function
blk_apply_bdi_limits() defaults to an initial setting of the ra_pages
field of struct backing_dev_info to VM_READAHEAD_PAGES, that is, 128 KB.
This low I/O size value is far from being optimal for hard-disk devices:
when reading files from multiple contexts using buffered I/Os, the seek
overhead between the small read commands generated to read-ahead
multiple files will significantly limit the performance that can be
achieved.
This fact applies to all ATA devices as ATA does not define an optimal
I/O size and the SCSI SAT specification does not define a default value
to expose to the host.
Modify blk_apply_bdi_limits() to use a device max_sectors limit to
calculate the ra_pages field of struct backing_dev_info, when the device
is a rotational one (BLK_FEAT_ROTATIONAL feature is set). For a SCSI
disk, this defaults to 2560 KB, which significantly improve performance
for buffered reads. Using XFS and sequentially reading randomly selected
(large) files stored on a SATA HDD, the maximum throughput achieved with
8 readers reading files with 1MB buffered I/Os increases from 122 MB/s
to 167 MB/s (+36%). The improvement is even larger when reading files
using 128 KB buffered I/Os, with a throughput increasing from 57 MB/s to
165 MB/s (+189%).
Signed-off-by: Damien Le Moal <dlemoal@xxxxxxxxxx>
---
block/blk-settings.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/block/blk-settings.c b/block/blk-settings.c
index a000daafbfb4..66d402de9026 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -58,16 +58,24 @@ EXPORT_SYMBOL(blk_set_stacking_limits);
void blk_apply_bdi_limits(struct backing_dev_info *bdi,
struct queue_limits *lim)
{
+ u64 io_opt = lim->io_opt;
+
/*
* For read-ahead of large files to be effective, we need to read ahead
- * at least twice the optimal I/O size.
+ * at least twice the optimal I/O size. For rotational devices that do
+ * not report an optimal I/O size (e.g. ATA HDDs), use the maximum I/O
+ * size to avoid falling back to the (rather inefficient) small default
+ * read-ahead size.
*
* There is no hardware limitation for the read-ahead size and the user
* might have increased the read-ahead size through sysfs, so don't ever
* decrease it.
*/
+ if (!io_opt && (lim->features & BLK_FEAT_ROTATIONAL))
+ io_opt = (u64)lim->max_sectors << SECTOR_SHIFT;
I'm still not sure it's even possible that lim->max_sectors <<
SECTOR_SHIFT overflows an unsigned int.
Anyway, FWIW:
Reviewed-by: John Garry <john.g.garry@xxxxxxxxxx>
