Subject: Re: 2.6.5-rc1-mm2 and direct_read_under and wb
From: Daniel McNeil <daniel@osdl.org>
To: Andrew Morton <akpm@osdl.org>
Cc: mason@suse.com, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       "linux-aio@kvack.org" <linux-aio@kvack.org>
In-Reply-To: <20040323095953.72786ccc.akpm@osdl.org>
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Date: 23 Mar 2004 13:38:01 -0800
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It looks like every place wbc->nonblocking is set to 1, sync_mode
is set to WB_SYNC_NONE, but there are places where WB_SYNC_NONE is
used and nonblocking is NOT set: 
	balance_dirty_pages()
	try_to_unuse()

So your patch makes balance_dirty_pages() do the lock_buffer()
in __block_write_full_page() instead of skipping and redirtying
the page.

I just making sure I understand.
So, WB_SYNC_ALL and nonblocking=1 should never be used?

Daniel

On Tue, 2004-03-23 at 09:59, Andrew Morton wrote:
> Daniel McNeil <daniel@osdl.org> wrote:
> >
> > Andrew,
> > 
> > Just to confirm my tests ran overnight without errors.
> 
> good.
> 
> > So the !wbc->nonblocking lowers the cpu utilization.
> 
> yup.  It prevents balance_dirty_pages() callers from spinning over the same
> pages again and again.
> 
> > What does sync_mode=WB_SYNC_ALL and nonblocking=1 mean?
> 
> WB_SYNC_ALL means "we're performing this writeout for data-integrity
> purposes, as opposed to for a regular dirty memory flush".  The code paths
> for these two things are deep, and are almost identical, so we pass it up
> and down in a flag.
> 
> ->nonblocking means "try to avoid blocking on request queues or locked
> pages".  pdflush and kswapd set this because those processes must serve
> many queues and cannot accord to get stuck on any one queue.  But regular
> write() callers do not set ->nonblocking because we _want_ these guys to
> throttle.
> 
> It's only OK to let pdflush avoid blocking on the buffer because pdflush
> will take an explicit nap in background_writeout().  hm, but only if the
> queue is congested - we may still have a problem there.

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