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SubjectRe: [PATCH 01/11] readahead: limit readahead size for small devices
On Wed, Feb 03, 2010 at 03:38:26AM +0800, Jens Axboe wrote:
> On Tue, Feb 02 2010, Wu Fengguang wrote:
> > 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
>
> I'm not sure the size part makes a ton of sense. You can have really
> fast small devices, and large slow devices. One real world example are
> the Sun FMod SSD devices, which are only 22GB in size but are faster
> than the Intel X25-E SLC disks.
>
> What makes it even worse for these devices is that they are often
> attached to fatter controllers than ahci, where command overhead is
> larger.

Ah, good to know about this fast 22GB SSD.

> Running your script on such a device yields (I enlarged the read-count
> by 2, makes it more reproducible):
>
> MARVELL SD88SA02 MP1F
>
> rasize 1st 2nd
> ------------------------------------------------------------------
> 4k 41 MB/s 41 MB/s
> 16k 85 MB/s 81 MB/s
> 32k 102 MB/s 109 MB/s
> 64k 125 MB/s 144 MB/s
> 128k 183 MB/s 185 MB/s
> 256k 216 MB/s 216 MB/s
> 512k 216 MB/s 236 MB/s
> 1024k 251 MB/s 252 MB/s
> 2M 258 MB/s 258 MB/s
> 4M 266 MB/s 266 MB/s
> 8M 266 MB/s 266 MB/s
>
> So for that device, 1M-2M looks like the sweet spot, with even needing
> 4-8M to fully reach full throughput.

Thanks for the data! I updated the formula to (16GB device => 1MB
readahead). However the limit in this patch is only true for <4GB
devices, since the default readahead size is merely 512KB.

IOW, this patch only limits the default readahead size (which is now
512KB in general and 4MB for btrfs). The user can always set any
readahead size.

> I don't think this is atypical of bigger systems. Only very recently
> have controller started to slim down the command overhead for real,
> because of the SSD devices. What probably is atypical is a device that
> is this small yet pretty fast.

Right. I didn't expect such small yet fast SSD..

Thanks,
Fengguang
---
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. This patch uses a formula
that generates the following concrete limits:

disk size readahead size
(scale by 4) (scale by 2)
1M 8k
4M 16k
16M 32k
64M 64k
256M 128k
1G 256k
--------------------------- (*)
4G 512k
16G 1024k
64G 2048k
256G 4096k
(*) Since the default readahead size is 512k, this limit only takes
effect for devices whose size is less than 4G.

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 4096 8192
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 (reported by Li Shaohua)

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

SSD 22G MARVELL SD88SA02 MP1F (reported by Jens Axboe)

rasize 1st 2nd
--------------------------------
4k 41 MB/s 41 MB/s
16k 85 MB/s 81 MB/s
32k 102 MB/s 109 MB/s
64k 125 MB/s 144 MB/s
128k 183 MB/s 185 MB/s
256k 216 MB/s 216 MB/s
512k 216 MB/s 236 MB/s
1024k 251 MB/s 252 MB/s
2M 258 MB/s 258 MB/s
==> 4M 266 MB/s 266 MB/s
8M 266 MB/s 266 MB/s
SSD 30G SanDisk SATA 5000

4k 29.6 MB/s 29.6 MB/s 29.6 MB/s
16k 52.1 MB/s 52.1 MB/s 52.1 MB/s
32k 61.5 MB/s 61.5 MB/s 61.5 MB/s
64k 67.2 MB/s 67.2 MB/s 67.1 MB/s
128k 71.4 MB/s 71.3 MB/s 71.4 MB/s
256k 73.4 MB/s 73.4 MB/s 73.3 MB/s
==> 512k 74.6 MB/s 74.6 MB/s 74.6 MB/s
1M 74.7 MB/s 74.6 MB/s 74.7 MB/s
2M 76.1 MB/s 74.6 MB/s 74.6 MB/s

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

4k 7.9 MB/s 7.9 MB/s 7.9 MB/s
16k 17.9 MB/s 17.9 MB/s 17.9 MB/s
32k 24.5 MB/s 24.5 MB/s 24.5 MB/s
64k 28.7 MB/s 28.7 MB/s 28.7 MB/s
128k 28.8 MB/s 28.9 MB/s 28.9 MB/s
==> 256k 30.5 MB/s 30.5 MB/s 30.5 MB/s
512k 30.9 MB/s 31.0 MB/s 30.9 MB/s
1M 31.0 MB/s 30.9 MB/s 30.9 MB/s
2M 30.9 MB/s 30.9 MB/s 30.9 MB/s

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

4k 6.4 MB/s 6.4 MB/s 6.4 MB/s
16k 13.4 MB/s 13.4 MB/s 13.2 MB/s
32k 17.8 MB/s 17.9 MB/s 17.8 MB/s
64k 21.3 MB/s 21.3 MB/s 21.2 MB/s
128k 21.4 MB/s 21.4 MB/s 21.4 MB/s
==> 256k 23.3 MB/s 23.2 MB/s 23.2 MB/s
512k 23.3 MB/s 23.8 MB/s 23.4 MB/s
1M 23.8 MB/s 23.4 MB/s 23.3 MB/s
2M 23.4 MB/s 23.2 MB/s 23.4 MB/s

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

4k 6.7 MB/s 6.9 MB/s 6.7 MB/s
16k 11.7 MB/s 11.7 MB/s 11.7 MB/s
32k 12.4 MB/s 12.4 MB/s 12.4 MB/s
64k 13.4 MB/s 13.4 MB/s 13.4 MB/s
128k 13.4 MB/s 13.4 MB/s 13.4 MB/s
==> 256k 13.6 MB/s 13.6 MB/s 13.6 MB/s
512k 13.7 MB/s 13.7 MB/s 13.7 MB/s
1M 13.7 MB/s 13.7 MB/s 13.7 MB/s
2M 13.7 MB/s 13.7 MB/s 13.7 MB/s

Anyone has 128MB USB stick? Anyway you get satisfiable performance
with >= 64k readahead size :)

CC: Jens Axboe <jens.axboe@oracle.com>
Tested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
block/genhd.c | 23 +++++++++++++++++++++++
1 file changed, 23 insertions(+)
--- linux.orig/block/genhd.c 2010-02-02 21:58:09.000000000 +0800
+++ linux/block/genhd.c 2010-02-03 13:57:54.000000000 +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 +552,28 @@ void add_disk(struct gendisk *disk)
retval = sysfs_create_link(&disk_to_dev(disk)->kobj, &bdi->dev->kobj,
"bdi");
WARN_ON(retval);
+
+ /*
+ * Limit default readahead size for small devices.
+ * disk size readahead size
+ * 1M 8k
+ * 4M 16k
+ * 16M 32k
+ * 64M 64k
+ * 256M 128k
+ * 1G 256k
+ * ---------------------------
+ * 4G 512k
+ * 16G 1024k
+ * 64G 2048k
+ * 256G 4096k
+ * Since the default readahead size is 512k, this limit
+ * only takes effect for devices whose size is less than 4G.
+ */
+
+ size = get_capacity(disk) >> 9;
+ size = 1UL << (ilog2(size) / 2);
+ bdi->ra_pages = min(bdi->ra_pages, size);
}

EXPORT_SYMBOL(add_disk);

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