Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | Date | Tue, 29 Nov 2011 10:40:15 +0800 | From | Wu Fengguang <> | Subject | Re: [PATCH 4/8] readahead: record readahead patterns |
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On Mon, Nov 21, 2011 at 03:19:19PM -0800, Andrew Morton wrote: > On Mon, 21 Nov 2011 17:18:23 +0800 > Wu Fengguang <fengguang.wu@intel.com> wrote: > > > Record the readahead pattern in ra_flags and extend the ra_submit() > > parameters, to be used by the next readahead tracing/stats patches. > > > > 7 patterns are defined: > > > > pattern readahead for > > ----------------------------------------------------------- > > RA_PATTERN_INITIAL start-of-file read > > RA_PATTERN_SUBSEQUENT trivial sequential read > > RA_PATTERN_CONTEXT interleaved sequential read > > RA_PATTERN_OVERSIZE oversize read > > RA_PATTERN_MMAP_AROUND mmap fault > > RA_PATTERN_FADVISE posix_fadvise() > > RA_PATTERN_RANDOM random read > > It would be useful to spell out in full detail what an "interleaved > sequential read" is, and why a read is considered "oversized", etc. > The 'enum readahead_pattern' definition site would be a good place for > this.
Good point, here is the added comments:
/* * Which policy makes decision to do the current read-ahead IO? * * RA_PATTERN_INITIAL readahead window is initially opened, * normally when reading from start of file * RA_PATTERN_SUBSEQUENT readahead window is pushed forward * RA_PATTERN_CONTEXT no readahead window available, querying the * page cache to decide readahead start/size. * This typically happens on interleaved reads (eg. * reading pages 0, 1000, 1, 1001, 2, 1002, ...) * where one file_ra_state struct is not enough * for recording 2+ interleaved sequential read * streams. * RA_PATTERN_MMAP_AROUND read-around on mmap page faults * (w/o any sequential/random hints) * RA_PATTERN_BACKWARDS reverse reading detected * RA_PATTERN_FADVISE triggered by POSIX_FADV_WILLNEED or FMODE_RANDOM * RA_PATTERN_OVERSIZE a random read larger than max readahead size, * do max readahead to break down the read size * RA_PATTERN_RANDOM a small random read */
> > Note that random reads will be recorded in file_ra_state now. > > This won't deteriorate cache bouncing because the ra->prev_pos update > > in do_generic_file_read() already pollutes the data cache, and > > filemap_fault() will stop calling into us after MMAP_LOTSAMISS. > > > > --- linux-next.orig/include/linux/fs.h 2011-11-20 20:10:48.000000000 +0800 > > +++ linux-next/include/linux/fs.h 2011-11-20 20:18:29.000000000 +0800 > > @@ -951,6 +951,39 @@ struct file_ra_state { > > > > /* ra_flags bits */ > > #define READAHEAD_MMAP_MISS 0x000003ff /* cache misses for mmap access */ > > +#define READAHEAD_MMAP 0x00010000 > > Why leave a gap?
Never mind, it's now converted to a bit field :)
> And what is READAHEAD_MMAP anyway?
READAHEAD_MMAP will be set for mmap page faults.
> > +#define READAHEAD_PATTERN_SHIFT 28 > > Why 28?
Bits 28-32 are for READAHEAD_PATTERN.
Anyway it will be gone when breaking down the ra_flags fields into individual variables. > > +#define READAHEAD_PATTERN 0xf0000000 > > + > > +/* > > + * Which policy makes decision to do the current read-ahead IO? > > + */ > > +enum readahead_pattern { > > + RA_PATTERN_INITIAL, > > + RA_PATTERN_SUBSEQUENT, > > + RA_PATTERN_CONTEXT, > > + RA_PATTERN_MMAP_AROUND, > > + RA_PATTERN_FADVISE, > > + RA_PATTERN_OVERSIZE, > > + RA_PATTERN_RANDOM, > > + RA_PATTERN_ALL, /* for summary stats */ > > + RA_PATTERN_MAX > > +}; > > Again, the behaviour is all undocumented. I see from the code that > multiple flags can be set at the same time. So afacit a file can be > marked RANDOM and SUBSEQUENT at the same time, which seems oxymoronic.
Nope, it will be classified into one "pattern" exclusively.
> This reader wants to know what the implications of this are - how the > code chooses, prioritises and acts. But this code doesn't tell me.
Hope the comment addresses this issue. The precise logic happens mainly inside ondemand_readahead().
> > +static inline unsigned int ra_pattern(unsigned int ra_flags) > > +{ > > + unsigned int pattern = ra_flags >> READAHEAD_PATTERN_SHIFT; > > OK, no masking is needed because the code silently assumes that arg > `ra_flags' came out of an ra_state.ra_flags and it also silently > assumes that no higher bits are used in ra_state.ra_flags. > > That's a bit of a handgrenade - if someone redoes the flags > enumeration, the code will explode.
Yeah sorry for playing with such tricks. Will get rid of this function totally and use a plain assign to ra->pattern.
> > + return min_t(unsigned int, pattern, RA_PATTERN_ALL); > > +} > > <scratches head> > > What the heck is that min_t() doing in there?
Just for safety... not really necessary given correct code.
Thanks, Fengguang
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