Messages in this thread |  | | | Date | Thu, 13 Mar 2003 11:31:13 +1000 | | From | Douglas Gilbert <> | | Subject | Re: Problem with aacraid driver in 2.5.63-bk-latest |
| |
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
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
|  |