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    SubjectRe: [Lse-tech] SCSI io_request_lock patch
    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.

    What problem you do see with exporting block internals?

    > o You are breaking SCSI merge completely, why on earth are you suddenly
    > using ll_*_merge functions for SCSI?!
    > o scsi_make_request need not worry about head active
    > o scsi_make_request can safe the q->*_merge indirect
    > o scsi_dispatch_cmd() io_request_lock removal looks racy

    I will investigate the above comments further.

    > At least you are not breaking anything other than SCSI this time...

    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?

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

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