a939888ec3
Add a new feature "zram idle/huge page writeback". In the zram-swap use
case, zram usually has many idle/huge swap pages. It's pointless to keep
them in memory (ie, zram).
To solve this problem, this feature introduces idle/huge page writeback to
the backing device so the goal is to save more memory space on embedded
systems.
Normal sequence to use idle/huge page writeback feature is as follows,
while (1) {
# mark allocated zram slot to idle
echo all > /sys/block/zram0/idle
# leave system working for several hours
# Unless there is no access for some blocks on zram,
# they are still IDLE marked pages.
echo "idle" > /sys/block/zram0/writeback
or/and
echo "huge" > /sys/block/zram0/writeback
# write the IDLE or/and huge marked slot into backing device
# and free the memory.
}
Per the discussion at
https://lore.kernel.org/lkml/20181122065926.GG3441@jagdpanzerIV/T/#u,
This patch removes direct incommpressibe page writeback feature
(d2afd25114f4 ("zram: write incompressible pages to backing device")).
Below concerns from Sergey:
== &< ==
"IDLE writeback" is superior to "incompressible writeback".
"incompressible writeback" is completely unpredictable and uncontrollable;
it depens on data patterns and compression algorithms. While "IDLE
writeback" is predictable.
I even suspect, that, *ideally*, we can remove "incompressible writeback".
"IDLE pages" is a super set which also includes "incompressible" pages.
So, technically, we still can do "incompressible writeback" from "IDLE
writeback" path; but a much more reasonable one, based on a page idling
period.
I understand that you want to keep "direct incompressible writeback"
around. ZRAM is especially popular on devices which do suffer from flash
wearout, so I can see "incompressible writeback" path becoming a dead
code, long term.
== &< ==
Below concerns from Minchan:
== &< ==
My concern is if we enable CONFIG_ZRAM_WRITEBACK in this implementation,
both hugepage/idlepage writeck will turn on. However someuser want to
enable only idlepage writeback so we need to introduce turn on/off knob
for hugepage or new CONFIG_ZRAM_IDLEPAGE_WRITEBACK for those usecase. I
don't want to make it complicated *if possible*.
Long term, I imagine we need to make VM aware of new swap hierarchy a
little bit different with as-is. For example, first high priority swap
can return -EIO or -ENOCOMP, swap try to fallback to next lower priority
swap device. With that, hugepage writeback will work tranparently.
So we could regard it as regression because incompressible pages doesn't
go to backing storage automatically. Instead, user should do it via "echo
huge" > /sys/block/zram/writeback" manually.
== &< ==
Link: http://lkml.kernel.org/r/20181127055429.251614-6-minchan@kernel.org
Signed-off-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Joey Pabalinas <joeypabalinas@gmail.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
116 lines
3.9 KiB
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116 lines
3.9 KiB
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What: /sys/block/zram<id>/disksize
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Date: August 2010
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Contact: Nitin Gupta <ngupta@vflare.org>
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Description:
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The disksize file is read-write and specifies the disk size
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which represents the limit on the *uncompressed* worth of data
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that can be stored in this disk.
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Unit: bytes
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What: /sys/block/zram<id>/initstate
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Date: August 2010
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Contact: Nitin Gupta <ngupta@vflare.org>
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Description:
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The initstate file is read-only and shows the initialization
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state of the device.
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What: /sys/block/zram<id>/reset
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Date: August 2010
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Contact: Nitin Gupta <ngupta@vflare.org>
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Description:
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The reset file is write-only and allows resetting the
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device. The reset operation frees all the memory associated
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with this device.
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What: /sys/block/zram<id>/max_comp_streams
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Date: February 2014
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Contact: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
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Description:
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The max_comp_streams file is read-write and specifies the
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number of backend's zcomp_strm compression streams (number of
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concurrent compress operations).
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What: /sys/block/zram<id>/comp_algorithm
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Date: February 2014
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Contact: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
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Description:
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The comp_algorithm file is read-write and lets to show
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available and selected compression algorithms, change
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compression algorithm selection.
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What: /sys/block/zram<id>/mem_used_max
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Date: August 2014
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Contact: Minchan Kim <minchan@kernel.org>
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Description:
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The mem_used_max file is write-only and is used to reset
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the counter of maximum memory zram have consumed to store
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compressed data. For resetting the value, you should write
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"0". Otherwise, you could see -EINVAL.
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Unit: bytes
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What: /sys/block/zram<id>/mem_limit
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Date: August 2014
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Contact: Minchan Kim <minchan@kernel.org>
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Description:
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The mem_limit file is write-only and specifies the maximum
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amount of memory ZRAM can use to store the compressed data.
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The limit could be changed in run time and "0" means disable
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the limit. No limit is the initial state. Unit: bytes
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What: /sys/block/zram<id>/compact
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Date: August 2015
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Contact: Minchan Kim <minchan@kernel.org>
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Description:
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The compact file is write-only and trigger compaction for
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allocator zrm uses. The allocator moves some objects so that
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it could free fragment space.
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What: /sys/block/zram<id>/io_stat
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Date: August 2015
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Contact: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
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Description:
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The io_stat file is read-only and accumulates device's I/O
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statistics not accounted by block layer. For example,
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failed_reads, failed_writes, etc. File format is similar to
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block layer statistics file format.
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What: /sys/block/zram<id>/mm_stat
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Date: August 2015
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Contact: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
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Description:
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The mm_stat file is read-only and represents device's mm
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statistics (orig_data_size, compr_data_size, etc.) in a format
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similar to block layer statistics file format.
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What: /sys/block/zram<id>/debug_stat
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Date: July 2016
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Contact: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
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Description:
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The debug_stat file is read-only and represents various
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device's debugging info useful for kernel developers. Its
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format is not documented intentionally and may change
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anytime without any notice.
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What: /sys/block/zram<id>/backing_dev
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Date: June 2017
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Contact: Minchan Kim <minchan@kernel.org>
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Description:
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The backing_dev file is read-write and set up backing
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device for zram to write incompressible pages.
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For using, user should enable CONFIG_ZRAM_WRITEBACK.
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What: /sys/block/zram<id>/idle
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Date: November 2018
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Contact: Minchan Kim <minchan@kernel.org>
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Description:
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idle file is write-only and mark zram slot as idle.
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If system has mounted debugfs, user can see which slots
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are idle via /sys/kernel/debug/zram/zram<id>/block_state
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What: /sys/block/zram<id>/writeback
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Date: November 2018
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Contact: Minchan Kim <minchan@kernel.org>
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Description:
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The writeback file is write-only and trigger idle and/or
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huge page writeback to backing device.
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