linux-kernel - Re: Disabling in-memory write cache for x86-64 in Linux II

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Message-ID: <20131104122613.GB24407@amd.pavel.ucw.cz>
Date:	Mon, 4 Nov 2013 13:26:13 +0100
From:	Pavel Machek <pavel@....cz>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	Theodore Ts'o <tytso@....edu>,
	"Artem S. Tashkinov" <t.artem@...os.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>,
	Maxim Patlasov <mpatlasov@...allels.com>
Subject: Re: Disabling in-memory write cache for x86-64 in Linux II

Hi!

> >>  - temp-files may not be written out at all.
> >>
> >>    Quite frankly, if you have multi-hundred-megabyte temptiles, you've
> >> got issues
> >   Actually people do stuff like this e.g. when generating ISO images before
> > burning them.
> 
> Yes, but then the temp-file is long-lived enough that it *will* hit
> the disk anyway. So it's only the "create temporary file and pretty
> much immediately delete it" case that changes behavior (ie compiler
> assembly files etc).
> 
> If the temp-file is for something like burning an ISO image, the
> burning part is slow enough that the temp-file will hit the disk
> regardless of when we start writing it.

It will hit the disk, but with proposed change, burning still will be
slower.

Before:

create 700MB iso
burn the CD, at the same time writing the iso to disk

After:

create 700MB iso and write most of it to disk
burn the CD, writing the rest.

But yes, limiting dirty ammounts is good idea.

> That said, I'd certainly like it even *more* if the limits really were
> per-BDI, and the global limit was in addition to the per-bdi ones.
> Because when you have a USB device that gets maybe 10MB/s on
> contiguous writes, and 100kB/s on random 4k writes, I think it would
> make more sense to make the "start writeout" limits be 1MB/2MB, not

Actually I believe I seen 10kB/sec on an SD card... would expect that
from USB sticks, too.

And yes, there are actually real problems with this at least on N900.

You do apt-get install <big package>. apt internally does fsyncs. It
results in big enough latencies that watchdogs kick in and kill the
machine.

http://pavelmachek.livejournal.com/117089.html

People are doing 

 echo 3 > /proc/sys/vm/dirty_ratio
    echo 3 > /proc/sys/vm/dirty_background_ratio
    echo 100 > /proc/sys/vm/dirty_writeback_centisecs 
    echo 100 > /proc/sys/vm/dirty_expire_centisecs 
    echo 4096 > /proc/sys/vm/min_free_kbytes 
    echo 50 > /proc/sys/vm/swappiness 
    echo 200 > /proc/sys/vm/vfs_cache_pressure 
    echo 8 > /proc/sys/vm/page-cluster
    echo 4 > /sys/block/mmcblk0/queue/nr_requests
    echo 4 > /sys/block/mmcblk1/queue/nr_requests

.. to avoid it, but IIRC it only makes the watchdog reset less likely
:-(.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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