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SubjectRe: [PATCH 5/9] readahead: on-demand readahead logic
On Fri, May 18, 2007 at 11:23:35PM -0700, Andrew Morton wrote:
> That would have to be the best changelog I've ever seen ;) Thanks for
> persisting with this.

Thank you :)

> > sysbench oltp (trans/sec): up to 8% gain
> Have you given any thought to identifying workloads which may be worsened
> by your changes? Attempt to deliberately expose any weak spots?

Yeah. All possible downsides I can imagine are:

- CPU overheads
Only random reads will be hurt.
That's 1% slow down for _sparse files_, and should be much smaller
when real I/O is involved.

- Behavior changes
It do not enforce strict check sequentialness.
- it is in general a good behavior for interleaved reads and
clustered-and-intermixed-random/sequential workloads.
- it might lead to more readahead misses
E.g. a random read sequence of 0,1,4,12,28,60,92,124,156,188,220
that is weird enough to start the readahead and hit all the
lookahead pages.
I highly doubt the possibility of such patterns happen in real
world. But if ever it happens repeatedly for some user, he can
work it around easily by tuning readahead_kb to some other value.

So, it is only a possibility that some random workload may be
worsened. But it's really hard to find one real world example.

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