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SubjectRe: [RFC] Make balance_dirty_pages zone aware (1/2)
"Martin J. Bligh" <mbligh@aracnet.com> wrote:
>
> "dd if=/dev/zero of=foo" would trigger it, I'd think. Watching the IO
> rate, it should go wierd after ram is full (on a 3 or more node system,
> so there's < 40% of RAM for each node).

Also, note that page_writeback_init() will not allow 40% of memory to be
dirtied on such a system. it is set much lower, partly to avoid an
explosion of unreclaimable buffer_heads.

bk revtool sez:


- Allowing 40% of physical memory to be dirtied on massive ia32 boxes
is unreasonable. It pins too many buffer_heads and contribues to
page reclaim latency.

The patch changes the initial value of
/proc/sys/vm/dirty_background_ratio, dirty_async_ratio and (the
presently non-functional) dirty_sync_ratio so that they are reduced
when the highmem:lowmem ratio exceeds 4:1.

These ratios are scaled so that as the highmem:lowmem ratio goes
beyond 4:1, the maximum amount of allowed dirty memory ceases to
increase. It is clamped at the amount of memory which a 4:1 machine
is allowed to use.

- Aggressive reduction in the dirty memory threshold at which
background writeback cuts in. 2.4 uses 30% of ZONE_NORMAL. 2.5 uses
40% of total memory. This patch changes it to 10% of total memory
(if total memory <= 4G. Even less otherwise - see above).

This means that:

- Much more writeback is performed by pdflush.

- When the application is generating dirty data at a moderate
rate, background writeback cuts in much earlier, so memory is
cleaned more promptly.

- Reduces the risk of user applications getting stalled by writeback.

- Will damage dbench numbers. It turns out that the damage is
fairly small, and dbench isn't a worthwhile workload for
optimisation.

- Moderate reduction in the dirty level at which the write(2) caller
is forced to perform writeback (throttling). Was 40% of total
memory. Is now 30% of total memory (if total memory <= 4G, less
otherwise).

This is to reduce page reclaim latency, and generally because
allowing processes to flood the machine with dirty data is a bad
thing in mixed workloads.

-
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