Re: 2.6.35 Regression: Ages spent discarding blocks that weren'tused!

From: Nigel Cunningham
Date: Wed Aug 04 2010 - 05:17:04 EST


On 04/08/10 18:59, Stefan Richter wrote:
(adding Cc: linux-scsi)

Nigel Cunningham wrote:
I've just given hibernation a go under 2.6.35, and at first I thought
there was some sort of hang in freezing processes. The computer sat
there for aaaaaages, apparently doing nothing. Switched from TuxOnIce to
swsusp to see if it was specific to my code but no - the problem was
there too. I used the nifty new kdb support to get a backtrace, which was:


Adding a printk in discard swap cluster gives the following:

[ 46.758330] Discarding 256 pages from bdev 800003 beginning at page 640377.
[ 47.003363] Discarding 256 pages from bdev 800003 beginning at page 640633.
[ 47.246514] Discarding 256 pages from bdev 800003 beginning at page 640889.


[ 221.877465] Discarding 256 pages from bdev 800003 beginning at page 826745.
[ 222.121284] Discarding 256 pages from bdev 800003 beginning at page 827001.
[ 222.365908] Discarding 256 pages from bdev 800003 beginning at page 827257.
[ 222.610311] Discarding 256 pages from bdev 800003 beginning at page 827513.

So allocating 4GB of swap on my SSD now takes 176 seconds instead of
virtually no time at all. (This code is completely unchanged from 2.6.34).

I have a couple of questions:

1) As far as I can see, there haven't been any changes in mm/swapfile.c
that would cause this slowdown, so something in the block layer has
(from my point of view) regressed. Is this a known issue?

Perhaps ATA TRIM is enabled for this SSD in 2.6.35 but not in 2.6.34?
Or the discard code has been changed to issue many moderately sized ATA
TRIMs instead of a single huge one, and the former was much more optimal
for your particular SSD?

Mmmm. Wonder how I tell. Something in dmesg or hdparm -I?

ata3.00: ATA-8: ARSSD56GBP, 1916, max UDMA/133
ata3.00: 500118192 sectors, multi 1: LBA48 NCQ (depth 31/32), AA
ata3.00: configured for UDMA/133
scsi 2:0:0:0: Direct-Access ATA ARSSD56GBP 1916 PQ: 0 ANSI: 5
sd 2:0:0:0: Attached scsi generic sg1 type 0
sd 2:0:0:0: [sda] 500118192 512-byte logical blocks: (256 GB/238 GiB)
sd 2:0:0:0: [sda] Write Protect is off
sd 2:0:0:0: [sda] Mode Sense: 00 3a 00 00
sd 2:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
sda: sda1 sda2 sda3 sda4
sd 2:0:0:0: [sda] Attached SCSI disk


ATA device, with non-removable media
Model Number: ARSSD56GBP
Serial Number: DC2210200F1B40015
Firmware Revision: 1916
Supported: 8 7 6 5
Likely used: 8
Logical max current
cylinders 16383 16383
heads 16 16
sectors/track 63 63
CHS current addressable sectors: 16514064
LBA user addressable sectors: 268435455
LBA48 user addressable sectors: 500118192
Logical Sector size: 512 bytes
Physical Sector size: 512 bytes
device size with M = 1024*1024: 244198 MBytes
device size with M = 1000*1000: 256060 MBytes (256 GB)
cache/buffer size = unknown
Nominal Media Rotation Rate: Solid State Device
LBA, IORDY(can be disabled)
Queue depth: 32
Standby timer values: spec'd by Standard, no device specific minimum
R/W multiple sector transfer: Max = 1 Current = 1
DMA: mdma0 mdma1 mdma2 udma0 udma1 udma2 udma3 udma4 udma5 *udma6
Cycle time: min=120ns recommended=120ns
PIO: pio0 pio1 pio2 pio3 pio4
Cycle time: no flow control=120ns IORDY flow control=120ns
Enabled Supported:
* SMART feature set
Security Mode feature set
* Power Management feature set
* Write cache
* Look-ahead
* Host Protected Area feature set
* WRITE_BUFFER command
* READ_BUFFER command
SET_MAX security extension
* 48-bit Address feature set
* Device Configuration Overlay feature set
* Mandatory FLUSH_CACHE
* SMART self-test
* General Purpose Logging feature set
* Gen1 signaling speed (1.5Gb/s)
* Gen2 signaling speed (3.0Gb/s)
* Native Command Queueing (NCQ)
* Phy event counters
* DMA Setup Auto-Activate optimization
Device-initiated interface power management
* Software settings preservation
* Data Set Management determinate TRIM supported
not enabled
not locked
not expired: security count
not supported: enhanced erase
Checksum: correct

2) Why are we calling discard_swap_cluster anyway? The swap was unused
and we're allocating it. I could understand calling it when freeing
swap, but when allocating?

At the moment when the administrator creates swap space, the kernel can
assume that he has no use anymore for the data that may have existed
previously at this space. Hence instruct the SSD's flash translation
layer to return all these blocks to the list of unused logical blocks
which do not have to be read and backed up whenever another logical
block within the same erase block is written to.

However, I am surprised that this is done every time (?) when preparing
for hibernation.

It's not hibernation per se. The discard code is called from a few places in swapfile.c in (afaict from a quick scan) both swap allocation and free paths.


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