Re: [PATCH] block: add max_dispatch to sysfs

[Jens Axboe <axboe@kernel.dk>]

On 4/10/24 4:18 AM, Dongliang Cui wrote:
> The default configuration in the current code is that when the device
> is not busy, a single dispatch will attempt to pull 'nr_requests'
> requests out of the schedule queue.
> 
> I tried to track the dispatch process:
> 
> COMM            TYPE    SEC_START       IOPRIO       INDEX
> fio-17304       R	196798040	0x2005	     0
> fio-17306       R	197060504	0x2005	     1
> fio-17307       R	197346904	0x2005	     2
> fio-17308       R	197609400	0x2005	     3
> fio-17309       R	197873048	0x2005	     4
> fio-17310       R	198134936	0x2005	     5
> ...
> fio-17237       R	197122936	  0x0	    57
> fio-17238       R	197384984	  0x0	    58
> <...>-17239     R	197647128	  0x0	    59
> fio-17240       R	197909208	  0x0	    60
> fio-17241       R	198171320	  0x0	    61
> fio-17242       R	198433432	  0x0	    62
> fio-17300       R	195744088	0x2005	     0
> fio-17301       R	196008504	0x2005	     0
> 
> The above data is calculated based on the block event trace, with each
> column containing: process name, request type, sector start address,
> IO priority.
> 
> The INDEX represents the order in which the requests are extracted from
> the scheduler queue during a single dispatch process.
> 
> Some low-speed devices cannot process these requests at once, and they will
> be requeued to hctx->dispatch and wait for the next issuance.
> 
> There will be a problem here, when the IO priority is enabled, if you try
> to dispatch "nr_request" requests at once, the IO priority will be ignored
> from the scheduler queue and all requests will be extracted.
> 
> In this scenario, if a high priority request is inserted into the scheduler
> queue, it needs to wait for the low priority request in the hctx->dispatch
> to be processed first.
> 
> --------------------dispatch 1st----------------------
> fio-17241       R       198171320         0x0       61
> fio-17242       R       198433432         0x0       62
> --------------------dispatch 2nd----------------------
> fio-17300       R       195744088       0x2005       0
> 
> In certain scenarios, we hope that requests can be processed in order of io
> priority as much as possible.
> 
> Maybe max_dispatch should not be a fixed value, but can be adjusted
> according to device conditions.
> 
> So we give a interface to control the maximum value of single dispatch
> so that users can configure it according to devices characteristics.

I agree that pulling 'nr_requests' out of the scheduler will kind of
defeat the purpose of the scheduler to some extent. But rather than add
another knob that nobody knows about or ever will touch (and extra queue
variables that just take up space), why not just default to something a
bit saner? Eg we could default to 1/8 or 1/4 of the scheduler depth
instead.

-- 
Jens Axboe