Re: Why is MEGASAS_SAS_QD set to 256?

[John Garry <john.g.garry@oracle.com>]

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?

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@huawei.com/

Thanks,
John