Re: When & how is the SCSI strategy routine called?

["lk" <linux_kernel@patni.com>]

There would be a single work queue (kblockd) for all block
devices on which deffered work of different block
device queues will be queued after the unplug_timer.


----- Original Message ----- 
From: "Rajat Jain, Noida" <rajatj@noida.hcltech.com>
To: "lk" <linux_kernel@patni.com>; <linux-scsi@vger.kernel.org>;
<rajatj@noida.hcltech.com>; <linux-newbie@vger.kernel.org>
Sent: Wednesday, March 23, 2005 12:49 AM
Subject: RE: When & how is the SCSI strategy routine called?


>
>
> Thanks lk.
>
> It cleared a lot of things.
>
> How ever, what I am wondering is:
>
> - Does each device queue have a corresponding thread that is woken up when
> it is time to call the request function? And the request function is
called
> in the context of this thread?
>
> - I explored and found that there is some "worker thread" kblockd that
does
> this task of monitoring the queue and calling the request function. I want
> to know whether there is a separate thread for each queue (device) , or
just
> one for all the block devices?
>
> TIA,
>
> Rajat
>
>
> -----Original Message-----
> From: lk [mailto:linux_kernel@patni.com]
> Sent: Wednesday, March 23, 2005 1:29 PM
> To: linux-scsi@vger.kernel.org; rajatj@noida.hcltech.com;
> linux-newbie@vger.kernel.org
> Subject: Re:When & how is the SCSI strategy routine called?
>
> This is my understanding.
> scsi_request_fn is the scsi's request function which performs the actual
> data transfer. clean up the queue and loop back to fetch next request.
>
> About the Question:WHEN and HOW is the strategy routine
"scsi_request_fn()"
> called ?
>
> Every request queue have a few members to decide whether to call
> scsi_request_fn() or not. scsi_request_fn is called after the queue is
> unplugged.
>
>   request_queue_t->unplug_thresh
>   Whenever request queue is about to get full. which is the number of
>   requests after which the queue should be unplugged. unplugging of
request
>   will in turn call scsi_request_fn().
>
>   request_queue_t->unplug_timer
>   After this much time the queue will be unplugged to rip the IO in turn
>   it will call scsi_request_fn().
>
>   Also in case of Async Direct IO the block layer will unplug the queue
> after
>   submission of  bios to queue by calling blk_run_address_space function.
>
>   scsi_request_fn will be called whenever the block layer calls queue
unplug
> or
>   queue is about to get full or after a certain unplug timer value.
>
>    Hope it helps.
>
>
> -----Original Message-----
> From: linux-scsi-owner@vger.kernel.org
> [mailto:linux-scsi-owner@vger.kernel.org]On Behalf Of Rajat Jain, Noida
> Sent: Tuesday, March 22, 2005 8:31 AM
> To: linux-scsi@vger.kernel.org; linux-newbie@vger.kernel.org
> Cc: Rajat Jain, Noida
> Subject: When & how is the SCSI strategy routine called?
>
>
> Hi list,
>
> Tracing the kernel 2.6.8 code I found that scsi_mod maintains separate
> request queues for each SCSI device. It uses the block layer queuing
> facility to do this. What I could not find out was that once a request is
> queued into a queue (for a particular device), WHEN and HOW is the
strategy
> routine "scsi_request_fn()" called ?
>
> All I could find on the net was that "The kernel calls the strategy
routine
> when ever it believes that it is appropriate to invoke it."
>
> Please help ... Any pointers will be highly appreciated.
>
> TIA,
>
> Rajat
>
>
>
>
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