• Sergey Senozhatsky's avatar
    zram: add multi stream functionality · beca3ec7
    Sergey Senozhatsky authored
    Existing zram (zcomp) implementation has only one compression stream
    (buffer and algorithm private part), so in order to prevent data
    corruption only one write (compress operation) can use this compression
    stream, forcing all concurrent write operations to wait for stream lock
    to be released.  This patch changes zcomp to keep a compression streams
    list of user-defined size (via sysfs device attr).  Each write operation
    still exclusively holds compression stream, the difference is that we
    can have N write operations (depending on size of streams list)
    executing in parallel.  See TEST section later in commit message for
    performance data.
    
    Introduce struct zcomp_strm_multi and a set of functions to manage
    zcomp_strm stream access.  zcomp_strm_multi has a list of idle
    zcomp_strm structs, spinlock to protect idle list and wait queue, making
    it possible to perform parallel compressions.
    
    The following set of functions added:
    - zcomp_strm_multi_find()/zcomp_strm_multi_release()
      find and release a compression stream, implement required locking
    - zcomp_strm_multi_create()/zcomp_strm_multi_destroy()
      create and destroy zcomp_strm_multi
    
    zcomp ->strm_find() and ->strm_release() callbacks are set during
    initialisation to zcomp_strm_multi_find()/zcomp_strm_multi_release()
    correspondingly.
    
    Each time zcomp issues a zcomp_strm_multi_find() call, the following set
    of operations performed:
    
    - spin lock strm_lock
    - if idle list is not empty, remove zcomp_strm from idle list, spin
      unlock and return zcomp stream pointer to caller
    - if idle list is empty, current adds itself to wait queue. it will be
      awaken by zcomp_strm_multi_release() caller.
    
    zcomp_strm_multi_release():
    - spin lock strm_lock
    - add zcomp stream to idle list
    - spin unlock, wake up sleeper
    
    Minchan Kim reported that spinlock-based locking scheme has demonstrated
    a severe perfomance regression for single compression stream case,
    comparing to mutex-based (see https://lkml.org/lkml/2014/2/18/16)
    
    base                      spinlock                    mutex
    
    ==Initial write           ==Initial write             ==Initial  write
    records:  5               records:  5                 records:   5
    avg:      1642424.35      avg:      699610.40         avg:       1655583.71
    std:      39890.95(2.43%) std:      232014.19(33.16%) std:       52293.96
    max:      1690170.94      max:      1163473.45        max:       1697164.75
    min:      1568669.52      min:      573429.88         min:       1553410.23
    ==Rewrite                 ==Rewrite                   ==Rewrite
    records:  5               records:  5                 records:   5
    avg:      1611775.39      avg:      501406.64         avg:       1684419.11
    std:      17144.58(1.06%) std:      15354.41(3.06%)   std:       18367.42
    max:      1641800.95      max:      531356.78         max:       1706445.84
    min:      1593515.27      min:      488817.78         min:       1655335.73
    
    When only one compression stream available, mutex with spin on owner
    tends to perform much better than frequent wait_event()/wake_up().  This
    is why single stream implemented as a special case with mutex locking.
    
    Introduce and document zram device attribute max_comp_streams.  This
    attr shows and stores current zcomp's max number of zcomp streams
    (max_strm).  Extend zcomp's zcomp_create() with `max_strm' parameter.
    `max_strm' limits the number of zcomp_strm structs in compression
    backend's idle list (max_comp_streams).
    
    max_comp_streams used during initialisation as follows:
    -- passing to zcomp_create() max_strm equals to 1 will initialise zcomp
    using single compression stream zcomp_strm_single (mutex-based locking).
    -- passing to zcomp_create() max_strm greater than 1 will initialise zcomp
    using multi compression stream zcomp_strm_multi (spinlock-based locking).
    
    default max_comp_streams value is 1, meaning that zram with single stream
    will be initialised.
    
    Later patch will introduce configuration knob to change max_comp_streams
    on already initialised and used zcomp.
    
    TEST
    iozone -t 3 -R -r 16K -s 60M -I +Z
    
           test           base       1 strm (mutex)     3 strm (spinlock)
    -----------------------------------------------------------------------
     Initial write      589286.78       583518.39          718011.05
           Rewrite      604837.97       596776.38         1515125.72
      Random write      584120.11       595714.58         1388850.25
            Pwrite      535731.17       541117.38          739295.27
            Fwrite     1418083.88      1478612.72         1484927.06
    
    Usage example:
    set max_comp_streams to 4
            echo 4 > /sys/block/zram0/max_comp_streams
    
    show current max_comp_streams (default value is 1).
            cat /sys/block/zram0/max_comp_streams
    Signed-off-by: default avatarSergey Senozhatsky <sergey.senozhatsky@gmail.com>
    Acked-by: default avatarMinchan Kim <minchan@kernel.org>
    Cc: Jerome Marchand <jmarchan@redhat.com>
    Cc: Nitin Gupta <ngupta@vflare.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    beca3ec7
zram_drv.h 2.71 KB