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

[David Lang]

On Fri, 25 Oct 2013, NeilBrown wrote:

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

actually, I think the problem is more the impact of the huge write later on.

David Lang

> 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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