Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 03 Nov 2010 16:54:42 +0100 | From | Richard Kralovic <> | Subject | Re: CFQ and dm-crypt |
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On 11/03/10 04:23, Jeff Moyer wrote: >> > CFQ io scheduler relies on using task_struct current to determine which >> > process makes the io request. On the other hand, some dm modules (such >> > as dm-crypt) use separate threads for doing io. As CFQ sees only these >> > threads, it provides a very poor performance in such a case. >> > >> > IMHO the correct solution for this would be to store, for every io >> > request, the process that initiated it (and preserve this information >> > while the request is processed by device mapper). Would that be feasible? > Sure. Try the attached patch (still an rfc) and let us know how it > goes. In my environment, it speed up multiple concurrent buffered > readers. I wasn't able to do a full analysis via blktrace as 2.6.37-rc1 > seems to have broken blktrace support on my system.
Thanks for the patch. Unfortunately, I got a kernel panic quite soon after booting the patched kernel. I was not able to reproduce the panic in a virtual machine, so I had to manually note the backtrace, thus I apologize that it's incomplete:
Fatal exception in interupt. ... do_invalid_op cic_free_func 0x9d/0xb0 bio_endio 0x42/0x70 task_rq_lock try_to_wake_up invalid_op cic_free_func cfq_free_io_context put_io_context cfq_put_request ...
When I combined the patch with my previous hack on dm-crypt, it worked fine; so the problem apparently goes away if cfq sees the corret io context.
Moreover, I noticed in the sources that cfq still uses current task on many places. For example, the CPU scheduler settings are inherited if there is no io priority set. Hence I was wondering if it does not make more sense to store whole task_struct of the initiating process in bio, instead of just io_context?
Greets Richard
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