Re: [RFC PATCH V2 04/17] blk-mq: don't reserve tags for admin queue

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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



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