RE: [PATCH] mm: moving dirty pages balancing to pdfludh entirely

["Ananiev, Leonid I" <leonid.i.ananiev@intel.com>]

Nikita Danilov writes:
> Some people do, should they suffer? :-)
You - yes. You have used that example as an argument incorrectly.

> Not _all_, only nr_to_write of them
	Yes. User thread writes all dirty pages in the system calling
writeback_inodes() and after it tests
			if (pages_written >= write_chunk)
	or have the user thread wrote more than 32+32/2 pages?

> In current design each thread is responsible for write-out.
	Casual thread performs common system work; casual thread is
throttled or frozen by this work.
	That is why a constant write_chunk==32+32/2 is used but too
late.
	Infinite number of pdflush may be created in current design and
only after extra pdflush thread is exited because other pdflush
processes the device. I have seen 6 pdflush threads during benchmarking
while I have 1 disk only. That is why MAX_PDFLUSH_THREADS is needed in
current design. The patch adds testing to keep off extra pdflush threads
creating.

Summary:
Nikita Danilov writes:
> Wouldn't this interfere with current->backing_dev_info logic?
	Proved: the patch does not break that logic.
> Intent of this is to throttle writers, and reduce risk of running oom
	Proved: a generator of dirty pages is throttled after patching
too.
	The extra throttling is removed. It and parallelism make
performance benefit.
Nikita Danilov writes that in current pdflush thread
> performs page-out even if queue is congested
	Proved: New pdflush does the same.
> With your patch, this work is done from pdflush, and won't be
throttled
	Proved: pdflush is throttled by device congestion.
> when direct reclaim skips writing dirty pages from tail of the
inactive list
	Proved: direct reclaim does not skip inactive list in proposed
design.
> You propose to limit write-out concurrency by MAX_PDFLUSH_THREADS
	Proved: the patch adds the line which keeps off creating of
infinite number of pdflush thread. The max limit could be removed in a
next patch.

Leonid

-----Original Message-----
From: Nikita Danilov [mailto:nikita@clusterfs.com] 
Sent: Thursday, July 06, 2006 12:06 AM
To: Ananiev, Leonid I
Cc: Bret Towe; Linux Kernel Mailing List
Subject: Re: [PATCH] mm: moving dirty pages balancing to pdfludh
entirely

Ananiev, Leonid I writes:
 > Nikita Danilov writes:
 > > Exactly to the contrary: as I explained to you, if you have more
 > devices
 > > than pdflush threads
 > I do not believe that Bret Towe has more devices than
 > MAX_PDFLUSH_THREADS=8.

Some people do, should they suffer? :-)

 > 
 > > See how wbc.nr_to_write is set up by balance_dirty_pages().
 > It is number TO write but I said about number after what user has to
 > write-out all dirty pages. 

Not _all_, only nr_to_write of them:

			if (pages_written >= write_chunk)
				break;		/* We've done our duty
*/

 > 
 > > imagine that MAX_PDFLUSH_THREADS equals 1
 > Imagine that CONFIG_NR_CPUS=1 for smp.
 > Kernel has a lot of "big enough" constants.

Then why introduce more of them?

In current design each thread is responsible for write-out. This means
that write-out concurrency level scales together with the number of
writers. You propose to limit write-out concurrency by
MAX_PDFLUSH_THREADS. Obviously this is an artificial limit that will be
sub-optimal sometimes.


 > 
 > Leonid

Nikita.
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