Re: Why is MEGASAS_SAS_QD set to 256?

[Yu Kuai]

Hi,

在 2022/11/25 20:33, John Garry 写道:
> On 24/11/2022 03:45, Yu Kuai wrote:
> > Hi,
> >
> > While upgrading kernel from 4.19 to 5.10, I found that fio 1 thread 4k
> > sequential io performance is dropped(160Mib -> 100 Mib), root cause is
> > that queue_depth is changed from 64 to 256.
> >
> > commit 6e73550670ed1c07779706bb6cf61b99c871fc42
> > scsi: megaraid_sas: Update optimal queue depth for SAS and NVMe devices
> >
> > diff --git a/drivers/scsi/megaraid/megaraid_sas.h 
> > b/drivers/scsi/megaraid/megaraid_sas.h
> > index bd8184072bed..ddfbe6f6667a 100644
> > --- a/drivers/scsi/megaraid/megaraid_sas.h
> > +++ b/drivers/scsi/megaraid/megaraid_sas.h
> > @@ -2233,9 +2233,9 @@ enum MR_PD_TYPE {
> >
> >   /* JBOD Queue depth definitions */
> >   #define MEGASAS_SATA_QD        32
> > -#define MEGASAS_SAS_QD 64
> > +#define MEGASAS_SAS_QD 256
> >   #define MEGASAS_DEFAULT_PD_QD  64
> > -#define MEGASAS_NVME_QD                32
> > +#define MEGASAS_NVME_QD        64
> >
> >
> > And with the default nr_requests 256, 256 queue_depth will make the
> > elevator has no effect, specifically io can't be merged in this test
> > case. Hence it doesn't make sense to me to set default queue_depth to
> > 256.
> >
> > Is there any reason why MEGASAS_SAS_QD is changed to 64?
> >
> > Thanks,
> > Kuai
>
> Which type of drive do you use?

SAS SSDs

BTW, I also test with nvme as well, the default elevator is deadline and
queue_depth seems too small, and performance is far from optimal.

Current default values don't seem good to me... 😒

Thanks,
Kuai
> JFYI, in case missed, there was this discussion on SCSI queue depth a 
> while ago:
> https://lore.kernel.org/linux-scsi/4b50f067-a368-2197-c331-a8c981f5cd02@xxxxxxxxxx/ 
>
>
> Thanks,
> John
>
>
> .