Messages in this thread | | | Date | Thu, 13 Mar 2003 18:27:33 -0500 | From | Doug Ledford <> | Subject | Re: Problem with aacraid driver in 2.5.63-bk-latest |
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Douglas Gilbert wrote: > 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).
Correct.
> 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.
Not throttle logic, we simply have a struct list_head that we stick one command (per host) onto and should it ever need to be used, then in scsi_done() when we would normally free a command we are done with we instead stick it back on that list head. That way, memory pressure can't kill us, just slow us down.
> I think that comment (probably by Doug Ledford) refers > to the 2.5 series before the slab allocator was > introduced.
Yep.
-- Doug Ledford <dledford@redhat.com> 919-754-3700 x44233 Red Hat, Inc. 1801 Varsity Dr. Raleigh, NC 27606
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