On Sun, 2007-07-22 at 16:45 +0800, Fengguang Wu wrote:
> On Sat, Jul 21, 2007 at 11:00:08PM +0200, Peter Zijlstra wrote:
> > Scale the default max readahead size with the system memory size.
> >
> > Signed-off-by: Peter Zijlstra <[EMAIL PROTECTED]>
> > ---
> > block/ll_rw_blk.c | 2 +-
> > include/linux/fs.h | 1 +
> > mm/readahead.c | 32 ++++++++++++++++++++++++++++++++
> > 3 files changed, 34 insertions(+), 1 deletion(-)
> >
> > Index: linux-2.6/block/ll_rw_blk.c
> > ===================================================================
> > --- linux-2.6.orig/block/ll_rw_blk.c
> > +++ linux-2.6/block/ll_rw_blk.c
> > @@ -208,7 +208,7 @@ void blk_queue_make_request(request_queu
> > blk_queue_max_phys_segments(q, MAX_PHYS_SEGMENTS);
> > blk_queue_max_hw_segments(q, MAX_HW_SEGMENTS);
> > q->make_request_fn = mfn;
> > - q->backing_dev_info.ra_pages = (VM_MAX_READAHEAD * 1024) /
> > PAGE_CACHE_SIZE;
> > + bdi_ra_init(&q->backing_dev_info);
>
> fs/fuse/inode.c has another line to be converted.
Drad, right you are. Will grep a bit.
> > q->backing_dev_info.state = 0;
> > q->backing_dev_info.capabilities = BDI_CAP_MAP_COPY;
> > blk_queue_max_sectors(q, SAFE_MAX_SECTORS);
> > Index: linux-2.6/include/linux/fs.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/fs.h
> > +++ linux-2.6/include/linux/fs.h
> > @@ -1696,6 +1696,7 @@ extern long do_splice_direct(struct file
> >
> > extern void
> > file_ra_state_init(struct file_ra_state *ra, struct address_space
> > *mapping);
> > +extern void bdi_ra_init(struct backing_dev_info *bdi);
> > extern loff_t no_llseek(struct file *file, loff_t offset, int origin);
> > extern loff_t generic_file_llseek(struct file *file, loff_t offset, int
> > origin);
> > extern loff_t remote_llseek(struct file *file, loff_t offset, int origin);
> > Index: linux-2.6/mm/readahead.c
> > ===================================================================
> > --- linux-2.6.orig/mm/readahead.c
> > +++ linux-2.6/mm/readahead.c
> > @@ -42,6 +42,38 @@ file_ra_state_init(struct file_ra_state
> > }
> > EXPORT_SYMBOL_GPL(file_ra_state_init);
> >
> > +static unsigned long ra_pages;
> > +
> > +static __init int readahead_init(void)
> > +{
> > + /*
> > + * Scale the max readahead window with system memory
> > + *
> > + * 64M: 128K
> > + * 128M: 180K
> > + * 256M: 256K
> > + * 512M: 360K
> > + * 1G: 512K
> > + * 2G: 724K
> > + * 4G: 1024K
> > + * 8G: 1448K
> > + * 16G: 2048K
> > + */
> > + ra_pages = int_sqrt(totalram_pages/16);
> > + if (ra_pages > (2 << (20 - PAGE_SHIFT)))
> > + ra_pages = 2 << (20 - PAGE_SHIFT);
>
> We can elaborate on the numbers ;)
>
> How about the following rules?
> - limit it under 1MB: we have to consider latencies
readahead is done async and we have these cond_resched() things
sprinkled all over, no?
> - make them alignment-friendly, i.e. 128K, 256K, 512K, 1M.
Would that actually matter? but yeah, that seems like a sane suggestion.
roundup_pow_of_two() comes to mind.
> My original plan is to simply do the following:
>
> - #define VM_MAX_READAHEAD 128 /* kbytes */
> + #define VM_MAX_READAHEAD 512 /* kbytes */
Yeah, the trouble I have with that is that it might adversely affect
tiny systems (although the trash detection might mitigate that impact)
> I'd like to post some numbers to back-up the discussion:
>
> readahead readahead
> size miss
> 128K 38%
> 512K 45%
> 1024K 49%
>
> The numbers are measured on a fresh booted KDE desktop.
>
> The majority misses come from the larger mmap read-arounds.
the mmap code never gets into readahead unless madvise(MADV_SEQUENTIAL)
is used afaik.
> Sequential readahead hits are pretty high and not quite affected by
> the readahead size, thanks to its size ramp-up process.
>
> > +
> > + return 0;
> > +}
> > +
> > +subsys_initcall(readahead_init);
>
> Remove the global ra_pages and fold readahead_init() into bdi_ra_init()?
> bdi_ra_init will only be called several times I guess.
I guess we could, this just seemed like a proper setup where more things
could grow into.
-
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