Re: [PATCH 00/33] Adaptive read-ahead V12

[Wu Fengguang]

On Thu, May 25, 2006 at 08:44:15AM -0700, Andrew Morton wrote:
> These are nice-looking numbers, but one wonders.  If optimising readahead
> makes this much difference to postgresql performance then postgresql should
> be doing the readahead itself, rather than relying upon the kernel's
> ability to guess what the application will be doing in the future.  Because
> surely the database can do a better job of that than the kernel.
> 
> That would involve using posix_fadvise(POSIX_FADV_RANDOM) to disable kernel
> readahead and then using posix_fadvise(POSIX_FADV_WILLNEED) to launch
> application-level readahead.
> 
> Has this been considered or attempted?

There has been many lengthy debates in the postgresql mailing list,
and it seems that there has been _strong_ resistance to it.

IMHO, a best scheme would be
        - leave _obvious_ patterns to the kernel
                i.e. all kinds of (semi-)sequential reads
        - do fadvise() for _non-obvious_ patterns on _critical_ points
                i.e. the index scans

Wu
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/