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
