forked from luck/tmp_suning_uos_patched
c0265342bf
zRam as swap is useful for small memory device. However, swap means
those pages on zram are mostly cold pages due to VM's LRU algorithm.
Especially, once init data for application are touched for launching,
they tend to be not accessed any more and finally swapped out. zRAM can
store such cold pages as compressed form but it's pointless to keep in
memory. Better idea is app developers free them directly rather than
remaining them on heap.
This patch tell us last access time of each block of zram via "cat
/sys/kernel/debug/zram/zram0/block_state".
The output is as follows,
300 75.033841 .wh
301 63.806904 s..
302 63.806919 ..h
First column is zram's block index and 3rh one represents symbol (s:
same page w: written page to backing store h: huge page) of the block
state. Second column represents usec time unit of the block was last
accessed. So above example means the 300th block is accessed at
75.033851 second and it was huge so it was written to the backing store.
Admin can leverage this information to catch cold|incompressible pages
of process with *pagemap* once part of heaps are swapped out.
I used the feature a few years ago to find memory hoggers in userspace
to notify them what memory they have wasted without touch for a long
time. With it, they could reduce unnecessary memory space. However, at
that time, I hacked up zram for the feature but now I need the feature
again so I decided it would be better to upstream rather than keeping it
alone. I hope I submit the userspace tool to use the feature soon.
[akpm@linux-foundation.org: fix i386 printk warning]
[minchan@kernel.org: use ktime_get_boottime() instead of sched_clock()]
Link: http://lkml.kernel.org/r/20180420063525.GA253739@rodete-desktop-imager.corp.google.com
[akpm@linux-foundation.org: documentation tweak]
[akpm@linux-foundation.org: fix i386 printk warning]
[minchan@kernel.org: fix compile warning]
Link: http://lkml.kernel.org/r/20180508104849.GA8209@rodete-desktop-imager.corp.google.com
[rdunlap@infradead.org: fix printk formats]
Link: http://lkml.kernel.org/r/3652ccb1-96ef-0b0b-05d1-f661d7733dcc@infradead.org
Link: http://lkml.kernel.org/r/20180416090946.63057-5-minchan@kernel.org
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
272 lines
10 KiB
Plaintext
272 lines
10 KiB
Plaintext
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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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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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_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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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 incompressible pages, there is no memory saving with zram.
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Instead, with CONFIG_ZRAM_WRITEBACK, zram can write incompressible page
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to backing storage rather than keeping it in memory.
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User should set up backing device via /sys/block/zramX/backing_dev
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before disksize setting.
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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 ..h
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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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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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