From mboxrd@z Thu Jan 1 00:00:00 1970 From: Douglas Gilbert Subject: Re: Problem with aacraid driver in 2.5.63-bk-latest Date: Thu, 13 Mar 2003 11:31:13 +1000 Sender: linux-scsi-owner@vger.kernel.org Message-ID: <3E6FDF61.8060708@torque.net> References: <20030228133037.GB7473@jiffies.dk> <1047510381.12193.28.camel@markh1.pdx.osdl.net> <1047514681.23725.35.camel@irongate.swansea.linux.org.uk> <3E6FC8D6.7090005@torque.net> <1047517604.23902.39.camel@irongate.swansea.linux.org.uk> Reply-To: dougg@torque.net Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Return-path: List-Id: linux-scsi@vger.kernel.org To: Alan Cox Cc: Mark Haverkamp , Christoffer Hall-Frederiksen , linux-scsi@vger.kernel.org, Linux Kernel Mailing List , linux aacraid devel , dledford@redhat.com Alan Cox wrote: > On Wed, 2003-03-12 at 23:55, Douglas Gilbert wrote: > >> /* >> * Limit max queue depth on a single lun to 256 for now. Remember, >> * we allocate a struct scsi_command for each of these and keep it >> * around forever. Too deep of a depth just wastes memory. >> */ >> if(tags > 256) >> return; >>.... > > > I can see the memory consideration. However the thing can really handle big > queues well. Possibly we should be setting the queue to 512 / somefunction(volumes) > though to avoid the worst case overcommit here The situation is different between 2.4 and 2.5 ... In 2.4 the per device queue_depth is an unsigned char and that number of scsi_cmnd instances are pre-allocated in the scsi_build_commandblocks() function. So the worst case number of scsi_cmnd instances for all scsi devices is always available (at the expense of [wasted] ram). In 2.5 queue_depth is an unsigned short and a slab allocator called "scsi_cmd_cache" is used as required. There is some throttle logic (or at least it has been talked about) to make sure one scsi_cmnd instance per scsi device will always be available. I think that comment (probably by Doug Ledford) refers to the 2.5 series before the slab allocator was introduced. Doug Gilbert