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SubjectRe: [PATCH 1/8] block: limit default readahead size for small devices
On Mon, Nov 21, 2011 at 02:52:47PM -0800, Andrew Morton wrote:
> On Mon, 21 Nov 2011 17:18:20 +0800
> Wu Fengguang <fengguang.wu@intel.com> 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)
>
> I'm disturbed that the code did a 4 page (16kbyte?) readahead after an
> lseek. Given the high probability that the next read will occur after
> a second lseek, that's a mistake.

Agreed.

> Was an lseek to offset 0 special-cased?

Yup, as you see in the last patch :)

> > 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).
>
> Spose so. Obviously there are other characteristics which should be
> considered when choosing a readaahead size, but one of them can be disk
> size and that's what this change does.

The other characteristics includes memory size, disk throughput,
latency and (unfortunately) above all the real workload and its
sensitivity to IO latencies.

The disk throughput and latency would be better than disk size,
however the former is not that easy to measure. Disk size is readily
available w/o any cost, hence this patch.

> In a better world, userspace would run a
> work-out-what-readahead-size-to-use script each time a distro is
> installed and when new storage devices are added/detected. Userspace
> would then remember that readahead size for subsequent bootups.
>
> In the real world, we shovel guaranteed-to-be-wrong guesswork into the
> kernel and everyone just uses the results. Sigh.

Heh. Disk characters should be measurable with some efforts.
However I won't even try to guess the workload and user demands...

Practically, I would choose a meaningful default size (512KB or 1MB)
for the majority and reduce it on small memory/slow disk systems (this
patch does the easy work based on disk size).

> > --- linux-next.orig/block/genhd.c 2011-10-31 00:13:51.000000000 +0800
> > +++ linux-next/block/genhd.c 2011-11-18 11:27:08.000000000 +0800
> > @@ -623,6 +623,26 @@ void add_disk(struct gendisk *disk)
> > WARN_ON(retval);
> >
> > disk_add_events(disk);
> > +
> > + /*
> > + * 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
> > + */
> > + if (get_capacity(disk)) {
> > + unsigned long size = get_capacity(disk) >> 9;
>
> get_capacity() returns sector_t. This expression will overflow with a
> 2T disk. I'm not sure if we successfully support 2T disks on 32-bit
> machines, but changes like this will guarantee that we don't :)

Good catch! I'll change the type to size_t.

> > + size = 1UL << (ilog2(size) / 2);
>
> I think there's a rounddown_pow_of_two() hiding in that expression?

Yeah, added a line of comment for that.

> > + bdi->ra_pages = min(bdi->ra_pages, size);
>
> I don't have a clue why that min() is in there. It needs a comment,
> please.

It's actually explained in the comment by word "Limit", I'll change it
to "Scale down" to make it more obvious.

Thanks,
Fengguang
---
Subject: block: limit default readahead size for small devices
Date: Fri Nov 18 11:27:12 CST 2011
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 128k, this limit only takes
effect for devices whose size is less than 256M.

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

64 MB, USB full speed (collected by Clemens Ladisch)
Bus 003 Device 003: ID 08ec:0011 M-Systems Flash Disk Pioneers DiskOnKey

4KB: 139.339 s, 376 kB/s
16KB: 81.0427 s, 647 kB/s
32KB: 71.8513 s, 730 kB/s
==> 64KB: 67.3872 s, 778 kB/s
128KB: 67.5434 s, 776 kB/s
256KB: 65.9019 s, 796 kB/s
512KB: 66.2282 s, 792 kB/s
1024KB: 67.4632 s, 777 kB/s
2048KB: 69.9759 s, 749 kB/s

An unnamed SD card (Yakui):

4k 195.873 s, 5.5 MB/s
8k 123.425 s, 8.7 MB/s
16k 86.6425 s, 12.4 MB/s
32k 66.7519 s, 16.1 MB/s
==> 64k 58.5262 s, 18.3 MB/s
128k 59.3847 s, 18.1 MB/s
256k 59.3188 s, 18.1 MB/s
512k 59.0218 s, 18.2 MB/s

CC: Li Shaohua <shaohua.li@intel.com>
CC: Clemens Ladisch <clemens@ladisch.de>
Acked-by: Jens Axboe <axboe@kernel.dk>
Acked-by: Rik van Riel <riel@redhat.com>
Tested-by: Vivek Goyal <vgoyal@redhat.com>
Tested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
block/genhd.c | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)
--- linux-next.orig/block/genhd.c 2011-11-21 20:32:56.000000000 +0800
+++ linux-next/block/genhd.c 2011-11-23 20:11:23.000000000 +0800
@@ -578,6 +578,7 @@ exit:
void add_disk(struct gendisk *disk)
{
struct backing_dev_info *bdi;
+ size_t size;
dev_t devt;
int retval;

@@ -623,6 +624,25 @@ void add_disk(struct gendisk *disk)
WARN_ON(retval);

disk_add_events(disk);
+
+ /*
+ * Scale down default readahead size for small devices.
+ * disk size readahead size
+ * 1M 8k
+ * 4M 16k
+ * 16M 32k
+ * 64M 64k
+ * 255M 64k (the round down effect)
+ * 256M 128k
+ * 1G 256k
+ * 4G 512k
+ * 16G 1024k
+ */
+ size = get_capacity(disk);
+ if (size) {
+ size = 1 << (ilog2(size >> 9) / 2);
+ bdi->ra_pages = min(bdi->ra_pages, size);
+ }
}
EXPORT_SYMBOL(add_disk);


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