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

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Date:	Fri, 25 Oct 2013 21:49:52 +1100
From:	NeilBrown <neilb@...e.de>
To:	"Artem S. Tashkinov" <t.artem@...os.com>
Cc:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org, axboe@...nel.dk, linux-mm@...ck.org
Subject: Re: Disabling in-memory write cache for x86-64 in Linux II

On Fri, 25 Oct 2013 07:25:13 +0000 (UTC) "Artem S. Tashkinov"
<t.artem@...os.com> wrote:

> Hello!
> 
> On my x86-64 PC (Intel Core i5 2500, 16GB RAM), I have the same 3.11 kernel
> built for the i686 (with PAE) and x86-64 architectures. What's really troubling me
> is that the x86-64 kernel has the following problem:
> 
> When I copy large files to any storage device, be it my HDD with ext4 partitions
> or flash drive with FAT32 partitions, the kernel first caches them in memory entirely
> then flushes them some time later (quite unpredictably though) or immediately upon
> invoking "sync".
> 
> How can I disable this memory cache altogether (or at least minimize caching)? When
> running the i686 kernel with the same configuration I don't observe this effect - files get
> written out almost immediately (for instance "sync" takes less than a second, whereas
> on x86-64 it can take a dozen of _minutes_ depending on a file size and storage
> performance).

What exactly is bothering you about this?  The amount of memory used or the
time until data is flushed?

If the later, then /proc/sys/vm/dirty_expire_centisecs is where you want to
look.
This defaults to 30 seconds (3000 centisecs).
You could make it smaller (providing you also shrink
dirty_writeback_centisecs in a similar ratio) and the VM will flush out data
more quickly.

NeilBrown


> 
> I'm _not_ talking about disabling write cache on my storage itself (hdparm -W 0 /dev/XXX)
> - firstly this command is detrimental to the performance of my PC, secondly, it won't help
> in this instance.
> 
> Swap is totally disabled, usually my memory is entirely free.
> 
> My kernel configuration can be fetched here: https://bugzilla.kernel.org/show_bug.cgi?id=63531
> 
> Please, advise.
> 
> Best regards,
> 
> Artem 
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