Re: [PATCH 12/33] readahead: min/max sizes

[Nick Piggin]

Wu Fengguang wrote:

> - Enlarge VM_MAX_READAHEAD to 1024 if new read-ahead code is compiled in.
>  This value is no longer tightly coupled with the thrashing problem,
>  therefore constrained by it. The adaptive read-ahead logic merely takes
>  it as an upper bound, and will not stick to it under memory pressure.

I guess this size enlargement is one of the main reasons your
patchset improves performance in some cases.

There is currently some sort of thrashing protection in there.
Obviously you've found it to be unable to cope with some situations
and introduced a lot of really fancy stuff to fix it. Are these just
academic access patterns, or do you have real test cases that
demonstrate this failure (ie. can we try to incrementally improve
the current logic as well as work towards merging your readahead
rewrite?)

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