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Subject[PATCH 01/11] readahead: limit readahead size for small devices
Linus reports a _really_ small & slow (505kB, 15kB/s) USB device,
on which blkid runs unpleasantly slow. He manages to optimize the blkid
reads down to 1kB+16kB, but still kernel read-ahead turns it into 48kB.

lseek 0, read 1024 => readahead 4 pages (start of file)
lseek 1536, read 16384 => readahead 8 pages (page contiguous)

The readahead heuristics involved here are reasonable ones in general.
So it's good to fix blkid with fadvise(RANDOM), as Linus already did.

For the kernel part, Linus suggests:
So maybe we could be less aggressive about read-ahead when the size of
the device is small? Turning a 16kB read into a 64kB one is a big deal,
when it's about 15% of the whole device!

This looks reasonable: smaller device tend to be slower (USB sticks as
well as micro/mobile/old hard disks).

Given that the non-rotational attribute is not always reported, we can
take disk size as a max readahead size hint. We use a formula that
generates the following concrete limits:

disk size readahead size
(scale by 4) (scale by 2)
2M 4k
8M 8k
32M 16k
128M 32k
512M 64k
2G 128k
8G 256k
32G 512k
128G 1024k

The formula is determined on the following data, collected by script:

#!/bin/sh

# please make sure BDEV is not mounted or opened by others
BDEV=sdb

for rasize in 4 16 32 64 128 256 512 1024 2048
do
echo $rasize > /sys/block/$BDEV/queue/read_ahead_kb
time dd if=/dev/$BDEV of=/dev/null bs=4k count=102400
done

The principle is, the formula shall not limit readahead size to such a
degree that will impact some device's sequential read performance.

The Intel SSD is special in that its throughput increases steadily with
larger readahead size. However it may take years for Linux to increase
its default readahead size to 2MB, so we don't take it seriously in the
formula.

SSD 80G Intel x25-M SSDSA2M080

rasize first run time/throughput second run time/throughput
------------------------------------------------------------------
4k 3.40038 s, 123 MB/s 3.42842 s, 122 MB/s
8k 2.7362 s, 153 MB/s 2.74528 s, 153 MB/s
16k 2.59808 s, 161 MB/s 2.58728 s, 162 MB/s
32k 2.50488 s, 167 MB/s 2.49138 s, 168 MB/s
64k 2.12861 s, 197 MB/s 2.13055 s, 197 MB/s
128k 1.92905 s, 217 MB/s 1.93176 s, 217 MB/s
256k 1.75896 s, 238 MB/s 1.78963 s, 234 MB/s
512k 1.67357 s, 251 MB/s 1.69112 s, 248 MB/s
1M 1.62115 s, 259 MB/s 1.63206 s, 257 MB/s
==> 2M 1.56204 s, 269 MB/s 1.58854 s, 264 MB/s
4M 1.57949 s, 266 MB/s 1.57426 s, 266 MB/s

Note that ==> points to the readahead size that yields plateau throughput.

SSD 30G SanDisk SATA 5000

4k 14.1593 s, 29.6 MB/s 14.1699 s, 29.6 MB/s 14.1782 s, 29.6 MB/s
8k 8.05231 s, 52.1 MB/s 8.04463 s, 52.1 MB/s 8.04758 s, 52.1 MB/s
16k 6.81751 s, 61.5 MB/s 6.81564 s, 61.5 MB/s 6.8146 s, 61.5 MB/s
32k 6.24176 s, 67.2 MB/s 6.2438 s, 67.2 MB/s 6.24645 s, 67.1 MB/s
64k 5.87828 s, 71.4 MB/s 5.87858 s, 71.3 MB/s 5.87481 s, 71.4 MB/s
128k 5.71649 s, 73.4 MB/s 5.71804 s, 73.4 MB/s 5.72055 s, 73.3 MB/s
==> 256k 5.62466 s, 74.6 MB/s 5.62304 s, 74.6 MB/s 5.62114 s, 74.6 MB/s
512k 5.61532 s, 74.7 MB/s 5.62098 s, 74.6 MB/s 5.61818 s, 74.7 MB/s
1M 5.50888 s, 76.1 MB/s 5.6204 s, 74.6 MB/s 5.62281 s, 74.6 MB/s

USB stick 32G Teclast CoolFlash idVendor=1307, idProduct=0165

4k 53.1635 s, 7.9 MB/s 53.155 s, 7.9 MB/s 53.107 s, 7.9 MB/s
8k 23.4061 s, 17.9 MB/s 23.3955 s, 17.9 MB/s 23.4222 s, 17.9 MB/s
16k 17.1077 s, 24.5 MB/s 17.0909 s, 24.5 MB/s 17.0875 s, 24.5 MB/s
32k 14.6029 s, 28.7 MB/s 14.5913 s, 28.7 MB/s 14.5951 s, 28.7 MB/s
64k 14.5483 s, 28.8 MB/s 14.5344 s, 28.9 MB/s 14.5333 s, 28.9 MB/s
==> 128k 13.7497 s, 30.5 MB/s 13.7364 s, 30.5 MB/s 13.731 s, 30.5 MB/s
256k 13.5521 s, 30.9 MB/s 13.5415 s, 31.0 MB/s 13.5554 s, 30.9 MB/s
512k 13.5414 s, 31.0 MB/s 13.5631 s, 30.9 MB/s 13.5654 s, 30.9 MB/s
1M 13.574 s, 30.9 MB/s 13.5686 s, 30.9 MB/s 13.5667 s, 30.9 MB/s

USB stick 4G SanDisk Cruzer idVendor=0781, idProduct=5151

4k 65.3449 s, 6.4 MB/s 65.3759 s, 6.4 MB/s 65.3405 s, 6.4 MB/s
8k 31.2002 s, 13.4 MB/s 31.1914 s, 13.4 MB/s 31.6836 s, 13.2 MB/s
16k 23.5281 s, 17.8 MB/s 23.4705 s, 17.9 MB/s 23.5859 s, 17.8 MB/s
32k 19.6786 s, 21.3 MB/s 19.719 s, 21.3 MB/s 19.7548 s, 21.2 MB/s
64k 19.6219 s, 21.4 MB/s 19.6125 s, 21.4 MB/s 19.594 s, 21.4 MB/s
==> 128k 18.021 s, 23.3 MB/s 18.0527 s, 23.2 MB/s 18.0694 s, 23.2 MB/s
256k 17.978 s, 23.3 MB/s 17.6483 s, 23.8 MB/s 17.9324 s, 23.4 MB/s
512k 17.659 s, 23.8 MB/s 17.9403 s, 23.4 MB/s 17.986 s, 23.3 MB/s
1M 17.9437 s, 23.4 MB/s 18.0634 s, 23.2 MB/s 17.9469 s, 23.4 MB/s

USB stick 2G idVendor=0204, idProduct=6025 SerialNumber: 08082005000113

4k 62.6246 s, 6.7 MB/s 60.5872 s, 6.9 MB/s 62.2581 s, 6.7 MB/s
8k 35.7505 s, 11.7 MB/s 35.764 s, 11.7 MB/s 35.7396 s, 11.7 MB/s
16k 33.7949 s, 12.4 MB/s 33.8041 s, 12.4 MB/s 33.8015 s, 12.4 MB/s
--> 32k 31.3851 s, 13.4 MB/s 31.381 s, 13.4 MB/s 31.3784 s, 13.4 MB/s
64k 31.3478 s, 13.4 MB/s 31.3494 s, 13.4 MB/s 31.3486 s, 13.4 MB/s
==> 128k 30.7384 s, 13.6 MB/s 30.7337 s, 13.6 MB/s 30.728 s, 13.6 MB/s
256k 30.5439 s, 13.7 MB/s 30.544 s, 13.7 MB/s 30.5433 s, 13.7 MB/s
512k 30.5408 s, 13.7 MB/s 30.543 s, 13.7 MB/s 30.5447 s, 13.7 MB/s
1M 30.5919 s, 13.7 MB/s 30.5893 s, 13.7 MB/s 30.5939 s, 13.7 MB/s

Anyone has 512/128MB USB stick? Anyway you get satisfiable performance
with >= 32k readahead size.

Tested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
block/genhd.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)

--- linux.orig/block/genhd.c 2010-01-21 21:17:16.000000000 +0800
+++ linux/block/genhd.c 2010-01-22 17:09:34.000000000 +0800
@@ -518,6 +518,7 @@ void add_disk(struct gendisk *disk)
struct backing_dev_info *bdi;
dev_t devt;
int retval;
+ unsigned long size;

/* minors == 0 indicates to use ext devt from part0 and should
* be accompanied with EXT_DEVT flag. Make sure all
@@ -551,6 +552,23 @@ void add_disk(struct gendisk *disk)
retval = sysfs_create_link(&disk_to_dev(disk)->kobj, &bdi->dev->kobj,
"bdi");
WARN_ON(retval);
+
+ /*
+ * limit readahead size for small devices
+ * disk size readahead size
+ * 2M 4k
+ * 8M 8k
+ * 32M 16k
+ * 128M 32k
+ * 512M 64k
+ * 2G 128k
+ * 8G 256k
+ * 32G 512k
+ * 128G 1024k
+ */
+ size = get_capacity(disk) >> 12;
+ size = 1UL << (ilog2(size) / 2);
+ bdi->ra_pages = min(bdi->ra_pages, size);
}

EXPORT_SYMBOL(add_disk);



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