From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Dillow Subject: Re: [dm-devel] multipath_busy() stalls IO due to scsi_host_is_busy() Date: Wed, 16 May 2012 13:03:33 -0400 Message-ID: <1337187813.21009.14.camel@frustration.ornl.gov> References: <4FB39D78.9020300@itwm.fraunhofer.de> <1337177200.2985.71.camel@dabdike.int.hansenpartnership.com> <4FB3B9DE.1050903@itwm.fraunhofer.de> <4FB3C75F.3070903@cs.wisc.edu> <4FB3CDC5.9040608@itwm.fraunhofer.de> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4FB3CDC5.9040608-mPn0NPGs4xGatNDF+KUbs4QuADTiUCJX@public.gmane.org> Sender: linux-rdma-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Bernd Schubert Cc: Mike Christie , device-mapper development , James Bottomley , "linux-scsi-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" List-Id: linux-scsi@vger.kernel.org On Wed, 2012-05-16 at 11:54 -0400, Bernd Schubert wrote: > 2) Low SRP command queues. Is there a reason why > SRP_RQ_SHIFT/SRP_RQ_SIZE and their depend values such as SRP_RQ_SIZE are > so small? That's a decision that has been around since the beginning of the driver as far as I can tell. It looks to be a balance between device needs and memory usage, and it can certainly be raised -- I've run locally with SRP_RQ_SHIFT set to 7 (shost.can_queue 126) and I'm sure 8 would be no problem, either. I didn't see a performance improvement on my workload, but may you will. Because we take the minimum of our initiator queue depth and the initial credits from the target (each req consumes a credit), going higher than 8 doesn't buy us much, as I don't know off-hand of any target that gives out more than 256 credits. The memory used for the command ring will vary depending on the value of SRP_RQ_SHIFT and the number of s/g entries allows to be put in the command. 255 s/g entries requires an 8 KB allocation for each request (~4200 bytes), so we currently require 512 KB of buffers for the send queue for each target. Going to 8 will require 2 MB max per target, which probably isn't a real issue. There's also a response ring with an allocation size that depends on the target, but those should be pretty small buffers, say 1 KB * (1 << SRP_RQ_SHIFT). -- Dave Dillow National Center for Computational Science Oak Ridge National Laboratory (865) 241-6602 office -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html