Messages in this thread | | | Date | Thu, 11 Dec 2008 22:00:58 +0300 | From | Vladislav Bolkhovitin <> | Subject | Re: [PATCH][RFC 13/23]: Export of alloc_io_context() function |
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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@vlnb.net> >>>> --- >>>> 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..
> 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 ;-)
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