linux-kernel - Re: Why is MEGASAS_SAS

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Date:   Sat, 26 Nov 2022 09:15:53 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     John Garry <john.g.garry@...cle.com>,
        Yu Kuai <yukuai1@...weicloud.com>, kashyap.desai@...adcom.com,
        sumit.saxena@...adcom.com, shivasharan.srikanteshwara@...adcom.com,
        jejb@...ux.ibm.com, martin.petersen@...cle.com,
        megaraidlinux.pdl@...adcom.com, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-block <linux-block@...r.kernel.org>,
        "zhangyi (F)" <yi.zhang@...wei.com>,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: Why is MEGASAS_SAS_QD set to 256?

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@huawei.com/ 
> 
> 
> Thanks,
> John
> 
> 
> .
>