Messages in this thread | | | Date | Wed, 25 Oct 2000 16:49:37 -0200 (BRDT) | From | Rik van Riel <> | Subject | Re: [PATCH] Re: PATCH: killing read_ahead[] |
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On Wed, 25 Oct 2000, Jeff V. Merkey wrote: > Rik van Riel wrote: > > On Wed, 25 Oct 2000, Jeff V. Merkey wrote: > > > > > I've reviewed the patch. It's affect seems minimal and will not > > > break NWFS as proposed -- it looks like, however, it will reduce > > > the performance slightly of EXT2/3 with iozone for read ahead > > > since the first section of the patch limits the read ahead > > > window size. > > > > Ummm, please read it again ;) > > > > The patch actually /increases/ the readahead size when > > we start to read a file from the beginning. > > But only if the file is smaller than MIN_READAHEAD * 2, which > would be the case for small files (which would read the whole > file anyway, which is how the page cache behaves today anyway).
If the file is bigger than MIN_READAHEAD * 2, we will want to read in the file in multiple IOs anyway.
If it turns out that we read that file sequentially, then the kernel will read in a LARGER CHUNK next time, if it turns out that we aren't using the file sequentially, we won't.
The point of chosing the MIN_READAHEAD * 2 cutoff is that I want to avoid the N+1 problem, where we do an IO for the first MIN_READAHEAD pages and have to do a separate IO for the last 1 page...
regards,
Rik -- "What you're running that piece of shit Gnome?!?!" -- Miguel de Icaza, UKUUG 2000
http://www.conectiva.com/ http://www.surriel.com/
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