Messages in this thread | | | Date | Thu, 9 Oct 2008 10:48:09 +0200 | From | Jens Axboe <> | Subject | Re: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority |
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On Wed, Oct 08 2008, Dave Chinner wrote: > On Tue, Oct 07, 2008 at 08:06:53PM +0200, Jens Axboe wrote: > > On Mon, Oct 06 2008, Dave Chinner wrote: > > > On Sat, Oct 04, 2008 at 05:45:00PM +1000, Aaron Carroll wrote: > > > > Dave Chinner wrote: > > > >> On Thu, Oct 02, 2008 at 05:32:04PM +0200, Bodo Eggert wrote: > > > >>> Sounds like you need a priority class besides sync and async. > > > >> > > > >> There's BIO_META now as well, which I was testing at the same time > > > >> as RT priority. Marking all the metadata I/O as BIO_META did help, > > > >> but once again I never got to determining if that was a result of > > > >> the different tagging or the priority increase. > > > > > > > > What exactly do you want META to mean? Strict prioritisation over > > > > all other non-META requests, or just more frequent and/or larger > > > > dispatches? Should META requests be sorted? > > > > > > The real question is "what was it supposed to mean"? AFAICT, it was > > > added to a couple of filesystems to be used to tag superblock read > > > I/O. Why - I don't know - there's a distinct lack of documentation > > > surrounding these bio flags. :/ > > > > It was added to be able to differentiate between data and meta data IO > > when using blktrace, that is all. > > Ok. > > > > Realistically, I'm not sure that having a separate queue for > > > BIO_META will buy us anything, given that noop is quite often the > > > fastest scheduler for XFS because it enables interleaved metadata > > > I/O to be merged with data I/O. Like I said, I was not able to spend > > > the time to determine exactly how BIO_META affected I/O patterns, so > > > I can't really comment on whether it is really necessary or not. > > > > There's no seperate queue for meta data IO anywhere. CFQ will give > > _slight_ preference to meta data IO as a side effect, preferring the > > meta IO for otherwise same IO in what to serve next in the same queue. > > And it will not allow preemption of a meta data IO for a data IO. > > > > So using meta should not yield any important boosts by itself. > > Which means that performance increase I saw on CFQ was a result of > removing the BIO_SYNC tagging "optimisation" XFS uses for metadata, > not from adding BIO_META..... > > Could you please document what these tags actually mean and do > so that other people don't get as confused as me about this > stuff....
Sure, I've added such a patch for 2.6.28.
-- Jens Axboe
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