Article about goals and implementation of Software Suspend for Linux
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Author: G\82ábor Kuti
+Author: G\82ábor Kuti
Last revised: 2003-10-20 by Pavel Machek
Idea and goals to achieve
website, and not to the Linux Kernel Mailing List. We are working
toward merging suspend2 into the mainline kernel.
-Q: A kernel thread must voluntarily freeze itself (call 'refrigerator').
-I found some kernel threads that don't do it, and they don't freeze
-so the system can't sleep. Is this a known behavior?
+Q: What is the freezing of tasks and why are we using it?
-A: All such kernel threads need to be fixed, one by one. Select the
-place where the thread is safe to be frozen (no kernel semaphores
-should be held at that point and it must be safe to sleep there), and
-add:
+A: The freezing of tasks is a mechanism by which user space processes and some
+kernel threads are controlled during hibernation or system-wide suspend (on some
+architectures). See freezing-of-tasks.txt for details.
- try_to_freeze();
-
-If the thread is needed for writing the image to storage, you should
-instead set the PF_NOFREEZE process flag when creating the thread (and
-be very carefull).
-
-
-Q: What is the difference between between "platform", "shutdown" and
-"firmware" in /sys/power/disk?
+Q: What is the difference between "platform" and "shutdown"?
A:
platform: save state in linux, then tell bios to powerdown and blink
"suspended led"
-firmware: tell bios to save state itself [needs BIOS-specific suspend
- partition, and has very little to do with swsusp]
-
-"platform" is actually right thing to do, but "shutdown" is most
-reliable.
+"platform" is actually right thing to do where supported, but
+"shutdown" is most reliable (except on ACPI systems).
Q: I do not understand why you have such strong objections to idea of
selective suspend.
-A: Do selective suspend during runtime power managment, that's okay. But
-its useless for suspend-to-disk. (And I do not see how you could use
+A: Do selective suspend during runtime power management, that's okay. But
+it's useless for suspend-to-disk. (And I do not see how you could use
it for suspend-to-ram, I hope you do not want that).
Lets see, so you suggest to
For devices like disk it does matter, you do not want to spindown for
FREEZE.
-Q: After resuming, system is paging heavilly, leading to very bad interactivity.
+Q: After resuming, system is paging heavily, leading to very bad interactivity.
A: Try running
suspend image to prevent sensitive data from being stolen after
resume.
-Q: Why can't we suspend to a swap file?
-
-A: Because accessing swap file needs the filesystem mounted, and
-filesystem might do something wrong (like replaying the journal)
-during mount.
-
-There are few ways to get that fixed:
-
-1) Probably could be solved by modifying every filesystem to support
-some kind of "really read-only!" option. Patches welcome.
+Q: Can I suspend to a swap file?
-2) suspend2 gets around that by storing absolute positions in on-disk
-image (and blocksize), with resume parameter pointing directly to
-suspend header.
+A: Generally, yes, you can. However, it requires you to use the "resume=" and
+"resume_offset=" kernel command line parameters, so the resume from a swap file
+cannot be initiated from an initrd or initramfs image. See
+swsusp-and-swap-files.txt for details.
Q: Is there a maximum system RAM size that is supported by swsusp?
A: If you want to see any non-error kernel messages on the virtual
terminal the kernel switches to during suspend, you have to set the
-kernel console loglevel to at least 5, for example by doing
-
- echo 5 > /proc/sys/kernel/printk
+kernel console loglevel to at least 4 (KERN_WARNING), for example by
+doing
+
+ # save the old loglevel
+ read LOGLEVEL DUMMY < /proc/sys/kernel/printk
+ # set the loglevel so we see the progress bar.
+ # if the level is higher than needed, we leave it alone.
+ if [ $LOGLEVEL -lt 5 ]; then
+ echo 5 > /proc/sys/kernel/printk
+ fi
+
+ IMG_SZ=0
+ read IMG_SZ < /sys/power/image_size
+ echo -n disk > /sys/power/state
+ RET=$?
+ #
+ # the logic here is:
+ # if image_size > 0 (without kernel support, IMG_SZ will be zero),
+ # then try again with image_size set to zero.
+ if [ $RET -ne 0 -a $IMG_SZ -ne 0 ]; then # try again with minimal image size
+ echo 0 > /sys/power/image_size
+ echo -n disk > /sys/power/state
+ RET=$?
+ fi
+
+ # restore previous loglevel
+ echo $LOGLEVEL > /proc/sys/kernel/printk
+ exit $RET
Q: Is this true that if I have a mounted filesystem on a USB device and
I suspend to disk, I can lose data unless the filesystem has been mounted
modes like "suspend-to-RAM" or "standby". (Don't write "disk" to the
/sys/power/state file; write "standby" or "mem".) We've not seen any
hardware that can use these modes through software suspend, although in
-theory some systems might support "platform" or "firmware" modes that
-won't break the USB connections.
+theory some systems might support "platform" modes that won't break the
+USB connections.
Remember that it's always a bad idea to unplug a disk drive containing a
mounted filesystem. That's true even when your system is asleep! The
Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays)
before suspending; then remount them after resuming.
+There is a work-around for this problem. For more information, see
+Documentation/usb/persist.txt.
+
+Q: Can I suspend-to-disk using a swap partition under LVM?
+
+A: No. You can suspend successfully, but you'll not be able to
+resume. uswsusp should be able to work with LVM. See suspend.sf.net.
+
+Q: I upgraded the kernel from 2.6.15 to 2.6.16. Both kernels were
+compiled with the similar configuration files. Anyway I found that
+suspend to disk (and resume) is much slower on 2.6.16 compared to
+2.6.15. Any idea for why that might happen or how can I speed it up?
+
+A: This is because the size of the suspend image is now greater than
+for 2.6.15 (by saving more data we can get more responsive system
+after resume).
+
+There's the /sys/power/image_size knob that controls the size of the
+image. If you set it to 0 (eg. by echo 0 > /sys/power/image_size as
+root), the 2.6.15 behavior should be restored. If it is still too
+slow, take a look at suspend.sf.net -- userland suspend is faster and
+supports LZF compression to speed it up further.
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff