Re: [PATCH] swapin readahead and fixes

Gerard Roudier (
Sat, 5 Dec 1998 11:47:41 +0100 (MET)

On Fri, 4 Dec 1998, Stephen C. Tweedie wrote:

> The idea is to do readahead for all the data you want, *including* the
> bit you are going to need right away. Once that is done, you just
> wait for the IO to complete on that first item. In this case, that
> means doing a readahead on pages n to n+15 inclusive, and then after
> that doing the synchronous read_swap_page on page n. The kernel will
> happily find that page in the swap cache, work out that IO is already
> in progress and wait for that page to become available.
> Even though the buffer IO request layer issues the entire sequential
> IO as one IO to the device drivers, the buffers and pages involved in
> the data transfer still get unlocked one by one as the IO completes.
> After submitting the initial IO you can wait for that first page to
> become unlocked without having to wait for the rest of the readahead
> IO to finish.

I find my previous reply to you mail very unclear. In fact, my idea is
to implemement something like the following:

- faulted_address
- offset_behind offset behind the faulted address up to which we want
to swap-in data.
- offset_ahead offset ahead the faulted address ...

- queue reads of all the pages between (faulted_address - offset_behind)
and (faulted_address + offset_ahead)
- run the tq_disk to actually start the IO.
- wait for the page that contains the faulted_address.

offset_behind and offset_ahead may be constant values or tuned at run-time
dynamically if some clever heuristic will be found.

Does the above make sense? (Or is it already implemented this way?)


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