From: Jens Axboe <axboe@kernel.dk>
To: John Garry <john.garry@huawei.com>,
Hannes Reinecke <hare@suse.de>,
"Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>,
James Bottomley <james.bottomley@hansenpartnership.com>,
Ming Lei <ming.lei@redhat.com>,
Bart van Assche <bvanassche@acm.org>,
linux-scsi@vger.kernel.org, linux-block@vger.kernel.org
Subject: Re: [PATCH 3/8] blk-mq: Use a pointer for sbitmap
Date: Tue, 26 Nov 2019 10:11:58 -0700 [thread overview]
Message-ID: <00a6d920-1855-c861-caa3-e845dcbe1fd8@kernel.dk> (raw)
In-Reply-To: <62838bca-cd3c-fccf-767c-76d8bea12324@huawei.com>
On 11/26/19 9:54 AM, John Garry wrote:
> On 26/11/2019 15:14, Jens Axboe wrote:
>> On 11/26/19 2:14 AM, Hannes Reinecke wrote:
>>> Instead of allocating the tag bitmap in place we should be using a
>>> pointer. This is in preparation for shared host-wide bitmaps.
>>
>> Not a huge fan of this, it's an extra indirection in the hot path
>> of both submission and completion.
>
> Hi Jens,
>
> Thanks for having a look.
>
> I checked the disassembly for blk_mq_get_tag() as a sample - which I
> assume is one hot path function which you care about - and the cost of
> the indirection is a load instruction instead of an add, denoted by ***,
> below:
I'm not that worried about an extra instruction, my worry is the extra
load is from different memory. When it's embedded in the struct, we're
on the same cache line or adjacent.
--
Jens Axboe
next prev parent reply other threads:[~2019-11-26 17:12 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-26 9:14 [PATCH RFC v3 0/8] blk-mq/scsi: Provide hostwide shared tags for SCSI HBAs Hannes Reinecke
2019-11-26 9:14 ` [PATCH 1/8] blk-mq: Remove some unused function arguments Hannes Reinecke
2019-11-26 9:14 ` [PATCH 2/8] blk-mq: rename BLK_MQ_F_TAG_SHARED as BLK_MQ_F_TAG_QUEUE_SHARED Hannes Reinecke
2019-11-26 9:14 ` [PATCH 3/8] blk-mq: Use a pointer for sbitmap Hannes Reinecke
2019-11-26 15:14 ` Jens Axboe
2019-11-26 16:54 ` John Garry
2019-11-26 17:11 ` Jens Axboe [this message]
2019-11-26 17:23 ` John Garry
2019-11-26 17:25 ` Jens Axboe
2019-11-26 18:08 ` John Garry
2019-11-27 1:46 ` Jens Axboe
2019-11-27 13:05 ` John Garry
2019-11-27 13:12 ` Hannes Reinecke
2019-11-27 14:20 ` Jens Axboe
2019-11-27 14:21 ` Jens Axboe
2019-11-27 14:44 ` John Garry
2019-11-27 16:52 ` Hannes Reinecke
2019-11-27 18:02 ` John Garry
2019-11-26 9:14 ` [PATCH 4/8] blk-mq: Facilitate a shared sbitmap per tagset Hannes Reinecke
2019-11-26 11:05 ` Ming Lei
2019-11-26 11:27 ` Hannes Reinecke
2019-11-26 11:50 ` John Garry
2019-11-26 15:54 ` Ming Lei
2019-11-27 17:02 ` Hannes Reinecke
2019-11-29 0:25 ` Ming Lei
2019-11-29 9:21 ` John Garry
2019-11-26 9:14 ` [PATCH 5/8] scsi: Add template flag 'host_tagset' Hannes Reinecke
2019-11-26 9:14 ` [PATCH 6/8] scsi: hisi_sas: Switch v3 hw to MQ Hannes Reinecke
2019-11-26 9:14 ` [PATCH 7/8] smartpqi: enable host tagset Hannes Reinecke
2019-11-26 9:14 ` [PATCH 8/8] hpsa: switch to using blk-mq Hannes Reinecke
2019-11-26 10:09 ` [PATCH RFC v3 0/8] blk-mq/scsi: Provide hostwide shared tags for SCSI HBAs John Garry
-- strict thread matches above, loose matches on Subject: below --
2019-11-26 13:10 [PATCH RFC v4 " Hannes Reinecke
2019-11-26 13:10 ` [PATCH 3/8] blk-mq: Use a pointer for sbitmap Hannes Reinecke
2019-11-26 16:42 ` Christoph Hellwig
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