On Thu, Dec 11 2008, Vladislav Bolkhovitin wrote: > Jens Axboe wrote: > >On Thu, Dec 11 2008, Vladislav Bolkhovitin wrote: > >>Jens Axboe wrote: > >>>On Wed, Dec 10 2008, Vladislav Bolkhovitin wrote: > >>>>This patch exports alloc_io_context() function. For performance reasons > >>>>SCST queues commands using a pool of IO threads. It is considerably > >>>>better for performance (>30% increase on sequential reads) if threads > >>>>in a pool have the same IO context. Since SCST can be built as a > >>>> module, it needs alloc_io_context() function exported. > >>>> > >>>>Signed-off-by: Vladislav Bolkhovitin <vst@xxxxxxxx> > >>>>--- > >>>> block/blk-ioc.c | 1 + > >>>> 1 file changed, 1 insertion(+) > >>>> > >>>>diff -upkr linux-2.6.27.2/block/blk-ioc.c linux-2.6.27.2/block/blk-ioc.c > >>>>--- linux-2.6.27.2/block/blk-ioc.c 2008-10-10 02:13:53.000000000 +0400 > >>>>+++ linux-2.6.27.2/block/blk-ioc.c 2008-11-25 21:27:01.000000000 +0300 > >>>>@@ -105,6 +105,7 @@ struct io_context *alloc_io_context(gfp_ > >>>> > >>>> return ret; > >>>>} > >>>>+EXPORT_SYMBOL(alloc_io_context); > >>>Why is this needed, can't you just use CLONE_IO? > >>There are two reasons for that: > >> > >>1. kthread interface doesn't support passing CLONE_IO flag. > > > >Then you fix that instead of working around it! :-) > > It doesn't worth the effort, because of (2) below. > > >>2. Each (virtual) device has own pool of threads, which serves it. > >>Threads in each such pools should have a common IO context, but > >>different pools should have different IO contexts. So, it would be > >>necessary to implement two levels start of IO threads in each pool. At > >>first, one thread would be started. Then it would call get_io_context() > >>to gain io_context. Then it would create the remaining threads with > >>CLONE_IO flag. Definitely, it's a lot more complicated than a simple > >>call of alloc_io_context() and assignment of the returned context to > >>each just created thread in a loop before they were ran. > > > >Just start the first thread without CLONE_IO, and subsequent threads > >fork off that with CLONE_IO set? > > Yes, that would be the two stages threads creation. A *LOT* more > complicated, than with the direct io_context assignment using > alloc_io_context(). > > >I think we need to make sure that we > >allocate an IO context for the 'parent' if it doesn't have one already > >and CLONE_IO is set, but that is something that can easily be rectified. > > Sorry, I don't feel I understood you here.. Sure I understand that it's then a two-stage rocket for the first context you fork off. I don't see how you qualify that as a *LOT* more complicated... > >It may seem more complex, but if you use this approach you are pretty > >much free to worry about any changes in the future there. > > Worrying about future changes is regular in Linux kernel, where there is > no stable API ;-) Sure, but if your stuff gets merged then *I* have to fiddle with your stuff as well when making changes. If you plan to keep your stuff out of the kernel and maintain it there, fine, but I think you probably don't. It's not a HUGE deal for this case, since you basically just want to use alloc_io_context() and ioc_task_link(). So we can make the export and be done with it. -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
