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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



Hi Ming

On 08/14/2018 10:10 AM, Ming Lei wrote:
> 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.

Yes.

> 
>>   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.
> 

I agree with you that the performance will not be affected.
But the adminq's budget here looks weird.
We don't reserve budget for admin queue (not count tag->active_queues for it).
But the admin queue has to comply to the limit in hctx_may_queue.

Since the there isn't many in-flight admin commands usually, we could take
admin queue here out of the limit of hctx_may_queue, then things could be clearer. :)

Thanks
Jianchao



[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[Index of Archives]     [SCSI Target Devel]     [Linux SCSI Target Infrastructure]     [Kernel Newbies]     [IDE]     [Security]     [Git]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Linux ATA RAID]     [Linux IIO]     [Samba]     [Device Mapper]

  Powered by Linux