From mboxrd@z Thu Jan 1 00:00:00 1970 From: "lk" Subject: Re:When & how is the SCSI strategy routine called? Date: Tue, 22 Mar 2005 23:58:54 -0800 Message-ID: <01a701c52f7e$290d9d30$5e91a8c0@patni.com> Reply-To: "lk" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-scsi-owner@vger.kernel.org List-Id: Content-Type: text/plain; charset="us-ascii" To: linux-scsi@vger.kernel.org, rajatj@noida.hcltech.com, linux-newbie@vger.kernel.org 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