On Mon, 1 Nov 1999 12:12:12 -0700, "Aaron J. Seigo" <aaron@gtv.ca> said:
> in response to a performance issue with disk i/o when using the
> postgresql database (maintained and developed mostly by bsd users,
> though used by many linux users) the following has come up...
> ____________________________________
>> Next question is what to do about it. I don't suppose we have any way
>> of turning off the OS' read-ahead algorithm :-(.
> Look what I found. I downloaded Linux kernel source for 2.2.0, and
> started looking for the word 'ahead' in the file system files. I found
> that read-ahead seems to be controlled by f_reada, and look where I
> found it being turned off? Seems like any seek turns off read-ahead on
> Linux.
It's a lot more complex than that --- check the do_generic_file_read()
code in mm/filemap.c for the full algorithm. The readahead "window" is
tuned dynamically based on sequential accesses, and any read outside the
window clears the readahead.
--Stephen
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