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    SubjectRe: [Lse-tech] SCSI io_request_lock patch
    Jens Axboe [axboe@suse.de] wrote:
    > On Tue, Nov 13 2001, Jonathan Lahr wrote:
    > > Jens Axboe [axboe@suse.de] wrote:
    > > > On Mon, Nov 12 2001, Jonathan Lahr wrote:
    > > > >
    > > > > This is a request for comments on the patch described below which
    > > > > implements a revised approach to reducing io_request_lock
    > > > > contention in 2.4.
    > > > >
    > > > > This new version of the io_request_lock patch (siorl-v0) is
    > > > > available at http://sourceforge.net/projects/lse/. It employs the
    > > > > same concurrent request queueing scheme as the iorlv0 patch but
    > > > > isolates code changes to the SCSI subsystem and engages the new
    > > > > locking scheme only for SCSI drivers which explicitly request it.
    > > > > I took this more restricted approach after additional development
    > > > > based on comments from Jens and others indicated that iorlv0
    > > > > impacted the IDE subsystem and was unnecessarily broad in general.
    > > > >
    > > > > The siorl-v0 patch allows drivers to enable concurrent queueing
    > > > > through the concurrent_queue field in the Scsi_Host_Template which
    > > > > is copied to the request queue. It creates SCSI-specific versions
    > > > > of generic block i/o functions used by the SCSI subsystem and
    > > > > modifies them to conditionally engage the new locking scheme based
    > > > > on this field. It allows control over which drivers use
    > > > > concurrent queueing and preserves original block i/o behavior by
    > > > > default.
    > > >
    > > > Sorry Jonathan, but this is even more broken than the last patch. In
    > > > different ways. In no particular order:
    > > >
    > > > o You are duplicating way too much code and exporting block
    > > > internals
    > >
    > > The duplication is a reasonable starting point for SCSI-specific
    > > functions. The block i/o design provides for exactly this type of
    > > tailoring through function pointers installed in request_queue.
    >
    > Yes I know, I wrote most of said code :-)

    And this approach makes good use of it.

    > > What problem you do see with exporting block internals?
    >
    > It's absolutely worthless. Look, it ties in with the points I made
    > below. You are exporting the merge functions for instance, and setting
    > them in the queue. This will cause scsi_merge not to use it's own
    > functions, broken.

    As in the baseline, initialize_merge_fn overwrites these pointers:
    q->back_merge_fn = scsi_back_merge_fn_;
    q->front_merge_fn = scsi_front_merge_fn_;
    q->merge_requests_fn = scsi_merge_requests_fn_;

    > > Do you think the separation of SCSI from generic block i/o code and
    > > the driver-activated control of concurrent queueing provides a path
    > > for future work to reduce io_request_lock contention in SCSI/FC?
    >
    > Not really, but I do think it could be a viable 2.4 alternative. For 2.5
    > we still want to do this the right way.

    I'll try to stay apprised of the 2.5 work as it progresses.

    --
    Jonathan Lahr
    IBM Linux Technology Center
    Beaverton, Oregon
    lahr@us.ibm.com
    503-578-3385

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