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    SubjectRe: Problem with aacraid driver in 2.5.63-bk-latest
    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).


    > 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.


    Doug Ledford <> 919-754-3700 x44233
    Red Hat, Inc.
    1801 Varsity Dr.
    Raleigh, NC 27606

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