Messages in this thread | | | Date | Sun, 22 Jul 2007 16:45:26 +0800 | From | Fengguang Wu <> | Subject | Re: [PATCH 3/3] readahead: scale max readahead size depending on memory size |
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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 <a.p.zijlstra@chello.nl> > --- > 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.
> 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 - make them alignment-friendly, i.e. 128K, 256K, 512K, 1M.
My original plan is to simply do the following:
- #define VM_MAX_READAHEAD 128 /* kbytes */ + #define VM_MAX_READAHEAD 512 /* kbytes */
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. 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.
> + > +void bdi_ra_init(struct backing_dev_info *bdi) > +{ > + bdi->ra_pages = ra_pages; > +} > +EXPORT_SYMBOL(bdi_ra_init);
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