[PATCH 03/15] readahead: bump up the default readahead size

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

[-- Attachment #1: readahead-enlarge-default-size.patch --]
[-- Type: text/plain, Size: 6813 bytes --]

Use 512kb max readahead size, and 32kb min readahead size.

The former helps io performance for common workloads.
The latter will be used in the thrashing safe context readahead.


====== Rationals on the 512kb size ======

I believe it yields more I/O throughput without noticeably increasing
I/O latency for today's HDD.

For example, for a 100MB/s and 8ms access time HDD, its random IO or
highly concurrent sequential IO would in theory be:

io_size KB  access_time  transfer_time  io_latency   util%   throughput KB/s
4           8             0.04           8.04        0.49%    497.57  
8           8             0.08           8.08        0.97%    990.33  
16          8             0.16           8.16        1.92%   1961.69 
32          8             0.31           8.31        3.76%   3849.62 
64          8             0.62           8.62        7.25%   7420.29 
128         8             1.25           9.25       13.51%  13837.84
256         8             2.50          10.50       23.81%  24380.95
512         8             5.00          13.00       38.46%  39384.62
1024        8            10.00          18.00       55.56%  56888.89
2048        8            20.00          28.00       71.43%  73142.86
4096        8            40.00          48.00       83.33%  85333.33

The 128KB => 512KB readahead size boosts IO throughput from ~13MB/s to
~39MB/s, while merely increases (minimal) IO latency from 9.25ms to 13ms.

As for SSD, I find that Intel X25-M SSD desires large readahead size
even for sequential reads:

	rasize	1st run		2nd run
	----------------------------------
	  4k	123 MB/s	122 MB/s
	 16k  	153 MB/s	153 MB/s
	 32k	161 MB/s	162 MB/s
	 64k	167 MB/s	168 MB/s
	128k	197 MB/s	197 MB/s
	256k	217 MB/s	217 MB/s
	512k	238 MB/s	234 MB/s
	  1M	251 MB/s	248 MB/s
	  2M	259 MB/s	257 MB/s
   	  4M	269 MB/s	264 MB/s
	  8M	266 MB/s	266 MB/s

The two other impacts of an enlarged readahead size are

- memory footprint (caused by readahead miss)
	Sequential readahead hit ratio is pretty high regardless of max
	readahead size; the extra memory footprint is mainly caused by
	enlarged mmap read-around.
	I measured my desktop:
	- under Xwindow:
		128KB readahead hit ratio = 143MB/230MB = 62%
		512KB readahead hit ratio = 138MB/248MB = 55%
		  1MB readahead hit ratio = 130MB/253MB = 51%
	- under console: (seems more stable than the Xwindow data)
		128KB readahead hit ratio = 30MB/56MB   = 53%
		  1MB readahead hit ratio = 30MB/59MB   = 51%
	So the impact to memory footprint looks acceptable.

- readahead thrashing
	It will now cost 1MB readahead buffer per stream.  Memory tight
	systems typically do not run multiple streams; but if they do
	so, it should help I/O performance as long as we can avoid
	thrashing, which can be achieved with the following patches.

I also boot the system into console with different readahead size,
and find that both the io_count and readahead_hit_ratio reduced by
~10% when increasing readahead_size from 128k to 512k. I guess typical
desktop users would prefer the reduced IO numbers (for fastboot) at
the cost of a dozen MB memory.

readahead_size	io_count   avg_io_pages   total_readahead_pages	 readahead_hit_ratio
            4k      6765              1    6765			 -
          128k      1077              8    8616			 78.5%
          512k       897             11    9867			 68.6%
         1024k       867             12   10404			 65.0%
total_readahead_pages = io_count * avg_io_size


====== Remarks by Christian Ehrhardt ======

- 512 is by far superior to 128 for sequential reads
- improvements with iozone sequential read scaling from 1 to 64 parallel
  processes up to +35%
- readahead sizes larger than 512 reevealed to not be "more useful" but
  increasing the chance of trashing in low mem systems


====== Benchmarks by Vivek Goyal ======

I have got two paths to the HP EVA and got multipath device setup(dm-3).
I run increasing number of sequential readers. File system is ext3 and
filesize is 1G.
I have run the tests 3 times (3sets) and taken the average of it.

Workload=bsr      iosched=cfq     Filesz=1G   bs=32K
======================================================================
                    2.6.33-rc5                2.6.33-rc5-readahead
job   Set NR  ReadBW(KB/s)   MaxClat(us)    ReadBW(KB/s)   MaxClat(us)
---   --- --  ------------   -----------    ------------   -----------
bsr   3   1   141768         130965         190302         97937.3    
bsr   3   2   131979         135402         185636         223286     
bsr   3   4   132351         420733         185986         363658     
bsr   3   8   133152         455434         184352         428478     
bsr   3   16  130316         674499         185646         594311     

I ran same test on a different piece of hardware. There are few SATA disks
(5-6) in striped configuration behind a hardware RAID controller.

Workload=bsr      iosched=cfq     Filesz=1G   bs=32K
======================================================================
                    2.6.33-rc5                2.6.33-rc5-readahead
job   Set NR  ReadBW(KB/s)   MaxClat(us)    ReadBW(KB/s)   MaxClat(us)    
---   --- --  ------------   -----------    ------------   -----------    
bsr   3   1   147569         14369.7        160191         22752          
bsr   3   2   124716         243932         149343         184698         
bsr   3   4   123451         327665         147183         430875         
bsr   3   8   122486         455102         144568         484045         
bsr   3   16  117645         1.03957e+06    137485         1.06257e+06    


CC: Jens Axboe <jens.axboe@oracle.com>
CC: Chris Mason <chris.mason@oracle.com>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
CC: Martin Schwidefsky <schwidefsky@de.ibm.com>
CC: Paul Gortmaker <paul.gortmaker@windriver.com>
CC: Matt Mackall <mpm@selenic.com>
CC: David Woodhouse <dwmw2@infradead.org>
Tested-by: Vivek Goyal <vgoyal@redhat.com>
Tested-by: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
Acked-by:  Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 include/linux/mm.h |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- linux.orig/include/linux/mm.h	2010-02-24 10:44:26.000000000 +0800
+++ linux/include/linux/mm.h	2010-02-24 10:44:41.000000000 +0800
@@ -1186,8 +1186,8 @@ int write_one_page(struct page *page, in
 void task_dirty_inc(struct task_struct *tsk);
 
 /* readahead.c */
-#define VM_MAX_READAHEAD	128	/* kbytes */
-#define VM_MIN_READAHEAD	16	/* kbytes (includes current page) */
+#define VM_MAX_READAHEAD	512	/* kbytes */
+#define VM_MIN_READAHEAD	32	/* kbytes (includes current page) */
 
 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
 			pgoff_t offset, unsigned long nr_to_read);