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From: Mike Christie <>
To: Ming Lei <>
Subject: Re: [RFC PATCH 1/4] scsi: Allow drivers to set BLK_MQ_F_BLOCKING
Date: Tue, 8 Mar 2022 19:17:13 -0600	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <Yif6jjlpPTEYpcAT@T590>

On 3/8/22 6:53 PM, Ming Lei wrote:
> On Mon, Mar 07, 2022 at 06:39:54PM -0600, Mike Christie wrote:
>> The software iscsi driver's queuecommand can block and taking the extra
>> hop from kblockd to its workqueue results in a performance hit. Allowing
>> it to set BLK_MQ_F_BLOCKING and transmit from that context directly
>> results in a 20-30% improvement in IOPs for workloads like:
>> fio --filename=/dev/sdb --direct=1 --rw=randrw --bs=4k --ioengine=libaio
>> --iodepth=128  --numjobs=1
>> and for all write workloads.
> This single patch shouldn't make any difference for iscsi, so please
> make it as last one if performance improvement data is provided
> in commit log.


> Also is there performance effect for other worloads? such as multiple
> jobs? iscsi is SQ hardware, so if driver is blocked in ->queuecommand()
> via BLK_MQ_F_BLOCKING, other contexts can't submit IO to scsi ML any more.

If you mean multiple jobs running on the same connection/session then
they are all serialized now. A connection can only do 1 cmd at a time.
There's a big mutex around it in the network layer, so multiple jobs
just suck no matter what.

If you mean multiple jobs from different connection/sessions, then the
iscsi code with this patchset blocks only because the network layer
takes a mutex for a short time. We configure it to not block for things
like socket space, memory allocations, we do zero copy IO normally, etc
so it's quick.

We also can do up to workqueues max_active limit worth of calls so
other things can normally send IO. We haven't found a need to increase
it yet.

  reply	other threads:[~2022-03-09  1:21 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-03-08  0:39 [RFC PATCH 0/4] scsi/iscsi: Send iscsi data from kblockd Mike Christie
2022-03-08  0:39 ` [RFC PATCH 1/4] scsi: Allow drivers to set BLK_MQ_F_BLOCKING Mike Christie
2022-03-08  4:57   ` Bart Van Assche
2022-03-09  0:53   ` Ming Lei
2022-03-09  1:17     ` Mike Christie [this message]
2022-03-09  1:43       ` Ming Lei
2022-03-09 19:38         ` Mike Christie
2022-03-08  0:39 ` [RFC PATCH 2/4] scsi: iscsi: Tell drivers when we must not block Mike Christie
2022-03-08  4:59   ` Bart Van Assche
2022-03-08 15:58     ` Mike Christie
2022-03-08  0:39 ` [RFC PATCH 3/4] scsi: iscsi: Support transmit from queuecommand Mike Christie
2022-03-08  0:39 ` [RFC PATCH 4/4] scsi: iscsi_tcp: Allow user to control if " Mike Christie
2022-03-08  5:10   ` Bart Van Assche
2022-03-08 16:51     ` Mike Christie
     [not found] ` <CGME20220308004023epcas2p12ebd497c14d32f36d0aa6682c0b9d0db@epcms2p7>
2022-03-08  7:00   ` [RFC PATCH 1/4] scsi: Allow drivers to set BLK_MQ_F_BLOCKING Daejun Park
2022-03-08 16:14     ` Mike Christie
2022-03-15  8:26 ` [RFC PATCH 0/4] scsi/iscsi: Send iscsi data from kblockd Christoph Hellwig
2022-03-16  1:08   ` Ming Lei

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