Re: [PATCH 1/3] block_write_full_page: Use synchronous writes for WBC_SYNC_ALL writebacks

Jens Axboe
2009-04-07T07:08:52Z

On Mon, Apr 06 2009, Andrew Morton wrote:
> On Mon, 6 Apr 2009 23:21:41 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:
>
> > I mean, let's graph it:
> >
> > WRITE_SYNC -> WRITE_SYNC_PLUG -> BIO_RW_SYNCIO -> bio_sync() -> REQ_RW_SYNC -> rw_is_sync() -> does something mysterious in get_request()
> > -> rq_is_sync() -> does something mysterious in IO schedulers
> > -> BIO_RW_NOIDLE -> bio_noidle() -> REQ_NOIDLE -> rq_noidle() -> does something mysterious in cfq-iosched only
> > -> BIO_RW_UNPLUG -> bio_unplug() -> REQ_UNPLUG -> OK, the cognoscenti know what this is supposed to do, but it is unused!
>
> whoop, I found a use of bio_unplug() in __make_request().
>
> So it appears that the intent of your patch is to cause an unplug after
> submission of each WB_SYNC_ALL block?
>
> But what about all the other stuff which WRITE_SYNC might or might not
> do? What does WRITE_SYNC _actually_ do, and what are the actual
> effects of this change??
>
> And what effect will this large stream of unplugs have upon merging?

It looks like a good candidate for WRITE_SYNC_PLUG instead, since it
does more than one buffer submission before waiting. It likely wont mean
a whole lot since we'll usually only have a single buffer on that page,
but for < PAGE_CACHE_SIZE block sizes it could easily make a big
difference (4 ios instead of 1!).

So on the write side, basically we have:

WRITE Normal async write.
WRITE_SYNC_PLUG Sync write, someone will wait on this so don't
treat it as background activity. This is a hint
to the io schedulers. This one does NOT unplug
the queue, either the caller should do it after
submission, or he should make sure that the
wait_on_* callbacks do it for him.
WRITE_SYNC Like WRITE_SYNC_PLUG, but causes immediate
unplug of the queue after submission. Most
uses of this should likely use WRITE_SYNC_PLUG,
at least in the normal IO path.
WRITE_ODIRECT Like WRITE_SYNC, but also passes a hint to the
IO scheduler that we should expect more IO.
This is similar to how a read is treated in the
scheduler, it'll enable anticipation/idling.

Ditto for the SWRITE* variants, which are special hacks for
ll_rw_block() only.

I have killed REQ_UNPLUG, it doesn't make sense to pass the further down
than to __make_request(), so the bio flag is enough.

--
Jens Axboe