Hi,
On Mon, 14 Feb 2000 18:00:22 -0800, Bruce Thompson
<bruce@otherother.com> said:
> Then I'm confused. If request overrun has been addressed then how
> can the oft quoted "cat /dev/zero > /foo/bar" trick cause a problem?
> Am I completely missing how this messes everything up?
Because these two things --- IO scheduling and writeback flow control
--- are done by completely different components of the system, and with
completely different purposes.
Write-behind, left unchecked, can obviously fill the whole of available
memory with dirty, pinned buffers. Flow-control on write-behind is
primarily a VM problem: we have to prevent dirty data from overwhelming
the caches. However, we may still quite legitimately find ourselves
with many MB of dirty data pending in the write queues before write
throttling kicks in.
That much dirty data can clearly overwhelm the fairness of a simple
CSCAN elevator scheduler, even when write throttling is working
correctly.
--Stephen
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