On 11/17/20 3:18 AM, Ming Lei wrote:
On Mon, Nov 16, 2020 at 10:44:54AM +0100, Hannes Reinecke wrote:
On 11/16/20 10:07 AM, Ming Lei wrote:
Limit scsi device's queue depth is less than max(host->can_queue, 1024)
in scsi_change_queue_depth(), and 1024 is big enough for saturating
current fast SCSI LUN(SSD, or raid volume on multiple SSDs).
We need this patch for replacing sdev->device_busy with sbitmap which
has to be pre-allocated with reasonable max depth.
Cc: Omar Sandoval <osandov@xxxxxx>
Cc: Kashyap Desai <kashyap.desai@xxxxxxxxxxxx>
Cc: Sumanesh Samanta <sumanesh.samanta@xxxxxxxxxxxx>
Cc: Ewan D. Milne <emilne@xxxxxxxxxx>
Tested-by: Sumanesh Samanta <sumanesh.samanta@xxxxxxxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
drivers/scsi/scsi.c | 11 +++++++++++
1 file changed, 11 insertions(+)
diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
index 24619c3bebd5..a28d48c850cf 100644
--- a/drivers/scsi/scsi.c
+++ b/drivers/scsi/scsi.c
@@ -214,6 +214,15 @@ void scsi_finish_command(struct scsi_cmnd *cmd)
scsi_io_completion(cmd, good_bytes);
}
+
+/*
+ * 1024 is big enough for saturating the fast scsi LUN now
+ */
+static int scsi_device_max_queue_depth(struct scsi_device *sdev)
+{
+ return max_t(int, sdev->host->can_queue, 1024);
+}
+
Shouldn't this rather be initialized with scsi_host->can_queue?
Multiple queues may be used for one single LUN, so in theory we should
return max queue depth as host->can_queue * host->nr_hw_queues, but
this number can be too big for the sbitmap's pre-allocation.
Ah, so that's the problem here.
That is why this patch introduces one reasonable limit on this value
of max(sdev->host->can_queue, 1024). Suppose single SSD can be saturated
by ~128 requests, we still can saturate one LUN with 8 SSDs behind if
the hw queue depth is set as too low.
These 'should be enough' settings inevitable turn out to be not enough in
the long run ...
I have provided the theory behind this idea, not just simple 'should be
enough'.
No, it's okay now. I wasn't aware that we had a limitation on the
sbitmap pre-allocation.
You can add my
Reviewed-by: Hannes Reinecke <hare@xxxxxxx>
Cheers,
Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
hare@xxxxxxx +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer
