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Date: Tue, 14 Aug 2018 10:10:51 +0800
From: Ming Lei <ming.lei@redhat.com>
To: "jianchao.wang" <jianchao.w.wang@oracle.com>
Cc: Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
	Alan Stern <stern@rowland.harvard.edu>,
	Christoph Hellwig <hch@lst.de>,
	Bart Van Assche <bart.vanassche@wdc.com>,
	Hannes Reinecke <hare@suse.de>,
	Johannes Thumshirn <jthumshirn@suse.de>,
	Adrian Hunter <adrian.hunter@intel.com>,
	"James E.J. Bottomley" <jejb@linux.vnet.ibm.com>,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	linux-scsi@vger.kernel.org
Subject: Re: [RFC PATCH V2 04/17] blk-mq: don't reserve tags for admin queue
Message-ID: <20180814021045.GA32541@ming.t460p>
References: <20180811071220.357-1-ming.lei@redhat.com>
 <20180811071220.357-5-ming.lei@redhat.com>
 <14c6e892-b05f-7422-0fd3-da45a645d3ba@oracle.com>
 <20180813104822.GA18557@ming.t460p>
 <85e5fb60-fdf1-436e-8bc1-5480d5b2b17f@oracle.com>
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On Tue, Aug 14, 2018 at 09:29:25AM +0800, jianchao.wang wrote:
> Hi Ming
> 
> On 08/13/2018 06:48 PM, Ming Lei wrote:
> > It is nothing to do with where the admin request is sent, so no any
> > difference wrt. this issue between with and without this patchset,
> > right?
> 
> I'm afraid not.
> 
> For example:
>   A scsi host has 8 LUNs associated with it.
>   Before this patch set,
>   When we send out the admin command, the budget is _per_ LUN, 1/8 of the total tags.
>   After this patch set,
>   When we send out the admin command, the budget is equal to _one_ LUN, 1/8 of the total tags.
> 
> However, the 1/8 above is different.
>   Before the patch set, every LUN's admin command has 1/8 budget to use which is per LUN.

Strictly speaking, it is that all admin command and all other IOs share the 1/8 budget
if they aimed at same LUN.

>   After this patch set, all the 8 LUNs admin command has to share the 1/8 budget.

That only means number of active admin commands won't be bigger than 1/8 budget, which
is one extra implicit limit on admin queue. However, other LUN's budget is still 1/8.

So performance for IO queue won't be affected at all, will it?

scsi_execute_* can't be called often, it is really in slow path, so I
don't think there is any possible performance effect with this patch, or do
you have other performance concern wrt. this patch?

We still have q->queue_depth for enhancing any limit for admin queue, but up to now,
not see it is necessary.

Thanks, 
Ming