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1d69a3f8ae
This patch includes some fixes and cleanup for idle-page writeback.
1. writeback_limit interface
Now writeback_limit interface is rather conusing. For example, once
writeback limit budget is exausted, admin can see 0 from
/sys/block/zramX/writeback_limit which is same semantic with disable
writeback_limit at this moment. IOW, admin cannot tell that zero came
from disable writeback limit or exausted writeback limit.
To make the interface clear, let's sepatate enable of writeback limit to
another knob - /sys/block/zram0/writeback_limit_enable
* before:
while true :
# to re-enable writeback limit once previous one is used up
echo 0 > /sys/block/zram0/writeback_limit
echo $((200<<20)) > /sys/block/zram0/writeback_limit
..
.. # used up the writeback limit budget
* new
# To enable writeback limit, from the beginning, admin should
# enable it.
echo $((200<<20)) > /sys/block/zram0/writeback_limit
echo 1 > /sys/block/zram/0/writeback_limit_enable
while true :
echo $((200<<20)) > /sys/block/zram0/writeback_limit
..
.. # used up the writeback limit budget
It's much strightforward.
2. fix condition check idle/huge writeback mode check
The mode in writeback_store is not bit opeartion any more so no need to
use bit operations. Furthermore, current condition check is broken in
that it does writeback every pages regardless of huge/idle.
3. clean up idle_store
No need to use goto.
[minchan@kernel.org: missed spin_lock_init]
Link: http://lkml.kernel.org/r/20190103001601.GA255139@google.com
Link: http://lkml.kernel.org/r/20181224033529.19450-1-minchan@kernel.org
Signed-off-by: Minchan Kim <minchan@kernel.org>
Suggested-by: John Dias <joaodias@google.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: John Dias <joaodias@google.com>
Cc: Srinivas Paladugu <srnvs@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
355 lines
13 KiB
Text
355 lines
13 KiB
Text
zram: Compressed RAM based block devices
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----------------------------------------
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* Introduction
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The zram module creates RAM based block devices named /dev/zram<id>
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(<id> = 0, 1, ...). Pages written to these disks are compressed and stored
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in memory itself. These disks allow very fast I/O and compression provides
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good amounts of memory savings. Some of the usecases include /tmp storage,
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use as swap disks, various caches under /var and maybe many more :)
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Statistics for individual zram devices are exported through sysfs nodes at
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/sys/block/zram<id>/
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* Usage
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There are several ways to configure and manage zram device(-s):
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a) using zram and zram_control sysfs attributes
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b) using zramctl utility, provided by util-linux (util-linux@vger.kernel.org).
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In this document we will describe only 'manual' zram configuration steps,
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IOW, zram and zram_control sysfs attributes.
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In order to get a better idea about zramctl please consult util-linux
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documentation, zramctl man-page or `zramctl --help'. Please be informed
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that zram maintainers do not develop/maintain util-linux or zramctl, should
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you have any questions please contact util-linux@vger.kernel.org
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Following shows a typical sequence of steps for using zram.
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WARNING
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=======
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For the sake of simplicity we skip error checking parts in most of the
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examples below. However, it is your sole responsibility to handle errors.
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zram sysfs attributes always return negative values in case of errors.
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The list of possible return codes:
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-EBUSY -- an attempt to modify an attribute that cannot be changed once
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the device has been initialised. Please reset device first;
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-ENOMEM -- zram was not able to allocate enough memory to fulfil your
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needs;
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-EINVAL -- invalid input has been provided.
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If you use 'echo', the returned value that is changed by 'echo' utility,
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and, in general case, something like:
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echo 3 > /sys/block/zram0/max_comp_streams
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if [ $? -ne 0 ];
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handle_error
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fi
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should suffice.
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1) Load Module:
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modprobe zram num_devices=4
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This creates 4 devices: /dev/zram{0,1,2,3}
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num_devices parameter is optional and tells zram how many devices should be
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pre-created. Default: 1.
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2) Set max number of compression streams
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Regardless the value passed to this attribute, ZRAM will always
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allocate multiple compression streams - one per online CPUs - thus
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allowing several concurrent compression operations. The number of
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allocated compression streams goes down when some of the CPUs
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become offline. There is no single-compression-stream mode anymore,
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unless you are running a UP system or has only 1 CPU online.
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To find out how many streams are currently available:
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cat /sys/block/zram0/max_comp_streams
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3) Select compression algorithm
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Using comp_algorithm device attribute one can see available and
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currently selected (shown in square brackets) compression algorithms,
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change selected compression algorithm (once the device is initialised
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there is no way to change compression algorithm).
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Examples:
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#show supported compression algorithms
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cat /sys/block/zram0/comp_algorithm
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lzo [lz4]
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#select lzo compression algorithm
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echo lzo > /sys/block/zram0/comp_algorithm
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For the time being, the `comp_algorithm' content does not necessarily
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show every compression algorithm supported by the kernel. We keep this
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list primarily to simplify device configuration and one can configure
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a new device with a compression algorithm that is not listed in
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`comp_algorithm'. The thing is that, internally, ZRAM uses Crypto API
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and, if some of the algorithms were built as modules, it's impossible
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to list all of them using, for instance, /proc/crypto or any other
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method. This, however, has an advantage of permitting the usage of
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custom crypto compression modules (implementing S/W or H/W compression).
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4) Set Disksize
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Set disk size by writing the value to sysfs node 'disksize'.
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The value can be either in bytes or you can use mem suffixes.
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Examples:
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# Initialize /dev/zram0 with 50MB disksize
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echo $((50*1024*1024)) > /sys/block/zram0/disksize
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# Using mem suffixes
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echo 256K > /sys/block/zram0/disksize
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echo 512M > /sys/block/zram0/disksize
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echo 1G > /sys/block/zram0/disksize
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Note:
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There is little point creating a zram of greater than twice the size of memory
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since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the
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size of the disk when not in use so a huge zram is wasteful.
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5) Set memory limit: Optional
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Set memory limit by writing the value to sysfs node 'mem_limit'.
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The value can be either in bytes or you can use mem suffixes.
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In addition, you could change the value in runtime.
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Examples:
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# limit /dev/zram0 with 50MB memory
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echo $((50*1024*1024)) > /sys/block/zram0/mem_limit
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# Using mem suffixes
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echo 256K > /sys/block/zram0/mem_limit
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echo 512M > /sys/block/zram0/mem_limit
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echo 1G > /sys/block/zram0/mem_limit
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# To disable memory limit
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echo 0 > /sys/block/zram0/mem_limit
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6) Activate:
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mkswap /dev/zram0
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swapon /dev/zram0
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mkfs.ext4 /dev/zram1
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mount /dev/zram1 /tmp
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7) Add/remove zram devices
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zram provides a control interface, which enables dynamic (on-demand) device
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addition and removal.
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In order to add a new /dev/zramX device, perform read operation on hot_add
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attribute. This will return either new device's device id (meaning that you
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can use /dev/zram<id>) or error code.
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Example:
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cat /sys/class/zram-control/hot_add
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1
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To remove the existing /dev/zramX device (where X is a device id)
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execute
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echo X > /sys/class/zram-control/hot_remove
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8) Stats:
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Per-device statistics are exported as various nodes under /sys/block/zram<id>/
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A brief description of exported device attributes. For more details please
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read Documentation/ABI/testing/sysfs-block-zram.
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Name access description
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---- ------ -----------
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disksize RW show and set the device's disk size
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initstate RO shows the initialization state of the device
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reset WO trigger device reset
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mem_used_max WO reset the `mem_used_max' counter (see later)
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mem_limit WO specifies the maximum amount of memory ZRAM can use
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to store the compressed data
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writeback_limit WO specifies the maximum amount of write IO zram can
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write out to backing device as 4KB unit
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writeback_limit_enable RW show and set writeback_limit feature
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max_comp_streams RW the number of possible concurrent compress operations
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comp_algorithm RW show and change the compression algorithm
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compact WO trigger memory compaction
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debug_stat RO this file is used for zram debugging purposes
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backing_dev RW set up backend storage for zram to write out
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idle WO mark allocated slot as idle
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User space is advised to use the following files to read the device statistics.
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File /sys/block/zram<id>/stat
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Represents block layer statistics. Read Documentation/block/stat.txt for
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details.
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File /sys/block/zram<id>/io_stat
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The stat file represents device's I/O statistics not accounted by block
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layer and, thus, not available in zram<id>/stat file. It consists of a
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single line of text and contains the following stats separated by
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whitespace:
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failed_reads the number of failed reads
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failed_writes the number of failed writes
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invalid_io the number of non-page-size-aligned I/O requests
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notify_free Depending on device usage scenario it may account
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a) the number of pages freed because of swap slot free
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notifications or b) the number of pages freed because of
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REQ_OP_DISCARD requests sent by bio. The former ones are
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sent to a swap block device when a swap slot is freed,
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which implies that this disk is being used as a swap disk.
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The latter ones are sent by filesystem mounted with
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discard option, whenever some data blocks are getting
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discarded.
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File /sys/block/zram<id>/mm_stat
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The stat file represents device's mm statistics. It consists of a single
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line of text and contains the following stats separated by whitespace:
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orig_data_size uncompressed size of data stored in this disk.
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This excludes same-element-filled pages (same_pages) since
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no memory is allocated for them.
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Unit: bytes
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compr_data_size compressed size of data stored in this disk
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mem_used_total the amount of memory allocated for this disk. This
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includes allocator fragmentation and metadata overhead,
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allocated for this disk. So, allocator space efficiency
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can be calculated using compr_data_size and this statistic.
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Unit: bytes
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mem_limit the maximum amount of memory ZRAM can use to store
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the compressed data
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mem_used_max the maximum amount of memory zram have consumed to
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store the data
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same_pages the number of same element filled pages written to this disk.
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No memory is allocated for such pages.
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pages_compacted the number of pages freed during compaction
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huge_pages the number of incompressible pages
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File /sys/block/zram<id>/bd_stat
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The stat file represents device's backing device statistics. It consists of
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a single line of text and contains the following stats separated by whitespace:
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bd_count size of data written in backing device.
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Unit: 4K bytes
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bd_reads the number of reads from backing device
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Unit: 4K bytes
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bd_writes the number of writes to backing device
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Unit: 4K bytes
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9) Deactivate:
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swapoff /dev/zram0
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umount /dev/zram1
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10) Reset:
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Write any positive value to 'reset' sysfs node
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echo 1 > /sys/block/zram0/reset
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echo 1 > /sys/block/zram1/reset
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This frees all the memory allocated for the given device and
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resets the disksize to zero. You must set the disksize again
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before reusing the device.
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* Optional Feature
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= writeback
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With CONFIG_ZRAM_WRITEBACK, zram can write idle/incompressible page
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to backing storage rather than keeping it in memory.
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To use the feature, admin should set up backing device via
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"echo /dev/sda5 > /sys/block/zramX/backing_dev"
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before disksize setting. It supports only partition at this moment.
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If admin want to use incompressible page writeback, they could do via
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"echo huge > /sys/block/zramX/write"
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To use idle page writeback, first, user need to declare zram pages
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as idle.
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"echo all > /sys/block/zramX/idle"
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From now on, any pages on zram are idle pages. The idle mark
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will be removed until someone request access of the block.
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IOW, unless there is access request, those pages are still idle pages.
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Admin can request writeback of those idle pages at right timing via
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"echo idle > /sys/block/zramX/writeback"
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With the command, zram writeback idle pages from memory to the storage.
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If there are lots of write IO with flash device, potentially, it has
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flash wearout problem so that admin needs to design write limitation
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to guarantee storage health for entire product life.
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To overcome the concern, zram supports "writeback_limit" feature.
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The "writeback_limit_enable"'s default value is 0 so that it doesn't limit
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any writeback. IOW, if admin want to apply writeback budget, he should
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enable writeback_limit_enable via
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$ echo 1 > /sys/block/zramX/writeback_limit_enable
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Once writeback_limit_enable is set, zram doesn't allow any writeback
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until admin set the budget via /sys/block/zramX/writeback_limit.
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(If admin doesn't enable writeback_limit_enable, writeback_limit's value
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assigned via /sys/block/zramX/writeback_limit is meaninless.)
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If admin want to limit writeback as per-day 400M, he could do it
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like below.
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$ MB_SHIFT=20
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$ 4K_SHIFT=12
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$ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \
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/sys/block/zram0/writeback_limit.
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$ echo 1 > /sys/block/zram0/writeback_limit_enable
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If admin want to allow further write again once the bugdet is exausted,
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he could do it like below
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$ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \
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/sys/block/zram0/writeback_limit
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If admin want to see remaining writeback budget since he set,
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$ cat /sys/block/zramX/writeback_limit
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If admin want to disable writeback limit, he could do
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$ echo 0 > /sys/block/zramX/writeback_limit_enable
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The writeback_limit count will reset whenever you reset zram(e.g.,
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system reboot, echo 1 > /sys/block/zramX/reset) so keeping how many of
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writeback happened until you reset the zram to allocate extra writeback
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budget in next setting is user's job.
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If admin want to measure writeback count in a certain period, he could
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know it via /sys/block/zram0/bd_stat's 3rd column.
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= memory tracking
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With CONFIG_ZRAM_MEMORY_TRACKING, user can know information of the
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zram block. It could be useful to catch cold or incompressible
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pages of the process with*pagemap.
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If you enable the feature, you could see block state via
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/sys/kernel/debug/zram/zram0/block_state". The output is as follows,
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300 75.033841 .wh.
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301 63.806904 s...
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302 63.806919 ..hi
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First column is zram's block index.
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Second column is access time since the system was booted
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Third column is state of the block.
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(s: same page
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w: written page to backing store
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h: huge page
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i: idle page)
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First line of above example says 300th block is accessed at 75.033841sec
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and the block's state is huge so it is written back to the backing
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storage. It's a debugging feature so anyone shouldn't rely on it to work
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properly.
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Nitin Gupta
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ngupta@vflare.org
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