Re: [PATCH 3/3] readahead: introduce context readahead algorithm

[Wu Fengguang <fengguang.wu@intel.com>]

On Tue, Apr 14, 2009 at 06:58:24PM +0800, Wu Fengguang wrote:
> On Tue, Apr 14, 2009 at 06:00:02PM +0800, Andi Kleen wrote:
> > > I'll list some possible situations. I guess you are referring to (2.3)?
> > 
> > Yep. Thanks for the detailed analysis.
> 
> You are welcome :-)
> 
> > > 2.3) readahead cache hits: rare case and the impact is temporary
> > > 
> > > The page at @offset-1 does get referenced by this stream, but it's
> > > created by someone else at some distant time ago. The page at
> > > @offset-1 may be lifted to active lru by this second reference, or too
> > > late and get reclaimed - by the time we reference page @offset.
> > > 
> > > Normally its a range of cached pages. We are a) either walking inside the
> > > range and enjoying the cache hits, b) or we walk out of it and restart
> > > readahead by ourself, c) or the range of cached pages get reclaimed
> > > while we are walking on them, and hence cannot find page @offset-1.
> > > 
> > > Obviously (c) is rare and temporary and is the main cause of (2.3).
> > > As soon as we goto the next page at @offset+1, we'll its 'previous'
> > > page at @offset to be cached(it is created by us!). So the context
> > > readahead starts working again - it's merely delayed by one page :-)
> > 
> > Thanks. The question is how much performance impact this has on
> > the stream that is readaheaded.  I guess it would be only a smaller
> > "hickup", with some luck hidden by the block level RA?

No the block level RA wont help here, because there are no disk
accesses at all for the cached pages before @offset: the disk RA have
absolutely no idea on where the IO for page @offset originates from ;-)

> Yes there will be hickup: whenever the stream walks out of the current
> cached page range(or the pages get reclaimed while we are walking in it),
> there will be a 1-page read, followed by a 4-page readahead, and then 8,
> 16, ... page sized readahead, i.e. a readahead window rampup process.
> 
> That's assuming we are doing 1-page reads. For large sendfile() calls,
> the readahead size will be instantly restored to its full size on the
> first cache miss.
> 
> > The other question would be if it could cause the readahead code
> > to do a lot of unnecessary work, but your answer seems to be "no". Fine.
> 
> Right. The readahead will automatically be turned off inside the
> cached page ranges, and restarted after walking out of it.
> 
> > I think the concept is sound.
> 
> ^_^ Thanks for your appreciation!
> 
> 
> Best Regards,
> Fengguang

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