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

[Wu Fengguang]

On Sat, Apr 11, 2009 at 08:16:52AM +0800, Andrew Morton wrote:
> On Fri, 10 Apr 2009 21:12:50 +0800
> Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> 
> > Introduce page cache context based readahead algorithm.
> > This is to better support concurrent read streams in general.
> > 
> > RATIONALE
> > ---------
> > The current readahead algorithm detects interleaved reads in a _passive_ way.
> > Given a sequence of interleaved streams 1,1001,2,1002,3,4,1003,5,1004,1005,6,...
> > By checking for (offset == prev_offset + 1), it will discover the sequentialness
> > between 3,4 and between 1004,1005, and start doing sequential readahead for the
> > individual streams since page 4 and page 1005.
> > 
> > The context readahead algorithm guarantees to discover the sequentialness no
> > matter how the streams are interleaved. For the above example, it will start
> > sequential readahead since page 2 and 1002.
> > 
> > The trick is to poke for page @offset-1 in the page cache when it has no other
> > clues on the sequentialness of request @offset: if the current requenst belongs
> > to a sequential stream, that stream must have accessed page @offset-1 recently,
> > and the page will still be cached now. So if page @offset-1 is there, we can
> > take request @offset as a sequential access.
> > 
> > BENEFICIARIES
> > -------------
> > - strictly interleaved reads  i.e. 1,1001,2,1002,3,1003,...
> >   the current readahead will take them as silly random reads;
> >   the context readahead will take them as two sequential streams.
> > 
> > - seeky _column_ iterations on a huge matrix
> >   Yes it can be regard as _massively_ interleaved streams!
> >   Context readahead could transform the 1-page IOs (@offset+@size):
> > 	0+1, 1000+1, 2000+1, 3000+1, ...,
> > 	1+1, 1001+1, 2001+1, 3001+1, ...,
> > 	2+1, 1002+1, 2002+1, 3002+1, ...
> >   into larger sized IOs:
> > 	0+1, 1000+1, 2000+1, 3000+1, ...,
> > 	1+4, 1001+4, 2001+4, 3001+4, ...,
> > 	5+8, 1005+8, 2005+8, 3005+8, ...
> > 
> > - cooperative IO processes   i.e. NFS and SCST
> >   They create a thread pool, farming off (sequential) IO requests to different
> >   threads which will be performing interleaved IO.
> > 
> >   It was not easy(or possible) to reliably tell from file->f_ra all those
> >   cooperative processes working on the same sequential stream, since they will
> >   have different file->f_ra instances. And NFSD's file->f_ra is particularly
> >   unusable, since their file objects are dynamically created for each request.
> >   The nfsd does have code trying to restore the f_ra bits, but not satisfactory.
> > 
> >   The new scheme is to detect the sequential pattern via looking up the page
> >   cache, which provides one single and consistent view of the pages recently
> >   accessed. That makes sequential detection for cooperative processes possible.
> > 
> > USER REPORT
> > -----------
> > Vladislav recommends the addition of context readahead as a result of his SCST
> > benchmarks. It leads to 6%~40% performance gains in various cases and achieves
> > equal performance in others.                http://lkml.org/lkml/2009/3/19/239
> > 
> > OVERHEADS
> > ---------
> > In theory, it introduces one extra page cache lookup per random read.  However
> > the below benchmark shows context readahead to be slightly faster, wondering..
> > 
> > Randomly reading 200MB amount of data on a sparse file, repeat 20 times for
> > each block size. The average throughputs are:
> > 
> >                        	original ra	context ra	gain
> >  4K random reads:	 65.561MB/s	 65.648MB/s	+0.1%
> > 16K random reads:	124.767MB/s	124.951MB/s	+0.1%
> > 64K random reads: 	162.123MB/s	162.278MB/s	+0.1%
> > 
> > Cc: Jens Axboe <jens.axboe@xxxxxxxxxx>
> > Cc: Jeff Moyer <jmoyer@xxxxxxxxxx>
> > Tested-by: Vladislav Bolkhovitin <vst@xxxxxxxx>
> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> 
> > ---
> >  mm/readahead.c |   60 +++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 60 insertions(+)
> > 
> > --- mm.orig/mm/readahead.c
> > +++ mm/mm/readahead.c
> > @@ -330,6 +330,59 @@ static unsigned long get_next_ra_size(st
> >   */
> >  
> >  /*
> > + * Count continuously cached pages from @offset-1 to @offset-@max,
> 
> You meant "contiguously" here, yes?

Ah yes, continuously for time and contiguously for space?

> > + * this count is a conservative estimation of
> > + * 	- length of the sequential read sequence, or
> > + * 	- thrashing threshold in memory tight systems
> > + */
> > +static unsigned long count_history_pages(struct address_space *mapping,
> > +					 struct file_ra_state *ra,
> > +					 pgoff_t offset, unsigned long max)
> > +{
> > +	pgoff_t head;
> > +
> > +	rcu_read_lock();
> > +	head = radix_tree_prev_hole(&mapping->page_tree, offset - 1, max);
> > +	rcu_read_unlock();
> > +
> > +	return offset - 1 - head;
> > +}
> 
> Doesn't matter much, but perhaps this should return pgoff_t.

Do you indicate to use pgoff_t for size?

> > +/*
> > + * page cache context based read-ahead
> > + */
> > +static int try_context_readahead(struct address_space *mapping,
> > +				 struct file_ra_state *ra,
> > +				 pgoff_t offset,
> > +				 unsigned long req_size,
> > +				 unsigned long max)
> > +{
> > +	unsigned long size;
> 
> And this could be pgoff_t too.

OK. I'll repost the whole series.

Thanks,
Fengguang
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