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

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Date:	Wed, 30 Oct 2013 10:07:08 +0000 (UTC)
From:	"Artem S. Tashkinov" <t.artem@...os.com>
To:	jack@...e.cz
Cc:	tytso@....edu, fengguang.wu@...el.com,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, diegocg@...il.com, david@...g.hm,
	neilb@...e.de
Subject: Re: Disabling in-memory write cache for x86-64 in Linux II

Oct 30, 2013 02:41:01 AM, Jack wrote:
On Fri 25-10-13 19:37:53, Ted Tso wrote:
>> Sure, although I wonder if it would be worth it calcuate some kind of
>> rolling average of the write bandwidth while we are doing writeback,
>> so if it turns out we got unlucky with the contents of the first 100MB
>> of dirty data (it could be either highly random or highly sequential)
>> the we'll eventually correct to the right level.
>  We already do average measured throughput over a longer time window and
>have kind of rolling average algorithm doing some averaging.
>
>> This means that VM would have to keep dirty page counters for each BDI
>> --- which I thought we weren't doing right now, which is why we have a
>> global vm.dirty_ratio/vm.dirty_background_ratio threshold.  (Or do I
>> have cause and effect reversed?  :-)
>  And we do currently keep the number of dirty & under writeback pages per
>BDI. We have global limits because mm wants to limit the total number of dirty
>pages (as those are harder to free). It doesn't care as much to which device
>these pages belong (although it probably should care a bit more because
>there are huge differences between how quickly can different devices get rid
>of dirty pages).

This might sound like an absolutely stupid question which makes no sense at
all, so I want to apologize for it in advance, but since the Linux kernel lacks
revoke(), does that mean that dirty buffers will always occupy the kernel memory
if I for instance remove my USB stick before the kernel has had the time to flush
these buffers?
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