Messages in this thread | | | Date | Wed, 6 Mar 2002 00:03:14 +0100 | From | Andrea Arcangeli <> | Subject | Re: 2.4.19pre1aa1 |
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On Tue, Mar 05, 2002 at 11:12:46AM -0800, Andrew Morton wrote: > Arjan van de Ven wrote: > > > > On Tue, Mar 05, 2002 at 07:26:04PM +0100, Andrea Arcangeli wrote: > > > > > Another approch would be to add the pages backing the bh into the lru > > > too, but then we'd need to mess with the slab and new bitflags, new > > > methods and so I don't think it's the best solution. The only good > > > reason for putting new kind of entries in the lru would be to age them > > > too the same way as the other pages, but we don't need that with the bh > > > (they're just in, and we mostly care only about the page age, not the bh > > > age). > > > > For 2.5 I kind of like this idea. There is one issue though: to make > > this work really well we'd probably need a ->prepareforfreepage() > > or similar page op (which for page cache pages can be equal to writepage() > > ) which the vm can use to prepare this page for freeing. > > If we stop using buffer_heads for pagecache I/O, we don't have this problem. > > I'm showing a 20% reduction in CPU load for large reads. Which is a *lot*, > given that read load is dominated by copy_to_user. > > 2.5 is significantly less efficient than 2.4 at this time. Some of that > seems to be due to worsened I-cache footprint, and a lot of it is due > to the way buffer_heads now have a BIO wrapper layer.
Indeed, at the moment bio is making the thing more expensive in CPU terms, even if OTOH it makes rawio fly.
> Take a look at submit_bh(). The writing is on the wall, guys. > > -
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