Messages in this thread |  | | Date | Wed, 14 Nov 2001 10:54:33 -0800 | From | Jonathan Lahr <> | Subject | Re: [Lse-tech] SCSI io_request_lock patch |
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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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