• Sergey Senozhatsky's avatar
    zram: introduce compressing backend abstraction · e7e1ef43
    Sergey Senozhatsky authored
    ZRAM performs direct LZO compression algorithm calls, making it the one
    and only option.  While LZO is generally performs well, LZ4 algorithm
    tends to have a faster decompression (see http://code.google.com/p/lz4/
    for full report)
    
    	Name            Ratio  C.speed D.speed
    	                        MB/s    MB/s
    	LZ4 (r101)      2.084    422    1820
    	LZO 2.06        2.106    414     600
    
    Thus, users who have mostly read (decompress) usage scenarious or mixed
    workflow (writes with relatively high read ops number) will benefit from
    using LZ4 compression backend.
    
    Introduce compressing backend abstraction zcomp in order to support
    multiple compression algorithms with the following set of operations:
    
            .create
            .destroy
            .compress
            .decompress
    
    Schematically zram write() usually contains the following steps:
    0) preparation (decompression of partioal IO, etc.)
    1) lock buffer_lock mutex (protects meta compress buffers)
    2) compress (using meta compress buffers)
    3) alloc and map zs_pool object
    4) copy compressed data (from meta compress buffers) to object allocated by 3)
    5) free previous pool page, assign a new one
    6) unlock buffer_lock mutex
    
    As we can see, compressing buffers must remain untouched from 1) to 4),
    because, otherwise, concurrent write() can overwrite data.  At the same
    time, zram_meta must be aware of a) specific compression algorithm memory
    requirements and b) necessary locking to protect compression buffers.  To
    remove requirement a) new struct zcomp_strm introduced, which contains a
    compress/decompress `buffer' and compression algorithm `private' part.
    While struct zcomp implements zcomp_strm stream handling and locking and
    removes requirement b) from zram meta.  zcomp ->create() and ->destroy(),
    respectively, allocate and deallocate algorithm specific zcomp_strm
    `private' part.
    
    Every zcomp has zcomp stream and mutex to protect its compression stream.
    Stream usage semantics remains the same -- only one write can hold stream
    lock and use its buffers.  zcomp_strm_find() turns caller into exclusive
    user of a stream (holding stream mutex until zram release stream), and
    zcomp_strm_release() makes zcomp stream available (unlock the stream
    mutex).  Hence no concurrent write (compression) operations possible at
    the moment.
    
    iozone -t 3 -R -r 16K -s 60M -I +Z
    
           test            base           patched
    --------------------------------------------------
      Initial write      597992.91       591660.58
            Rewrite      609674.34       616054.97
               Read     2404771.75      2452909.12
            Re-read     2459216.81      2470074.44
       Reverse Read     1652769.66      1589128.66
        Stride read     2202441.81      2202173.31
        Random read     2236311.47      2276565.31
     Mixed workload     1423760.41      1709760.06
       Random write      579584.08       615933.86
             Pwrite      597550.02       594933.70
              Pread     1703672.53      1718126.72
             Fwrite     1330497.06      1461054.00
              Fread     3922851.00      3957242.62
    
    Usage examples:
    
    	comp = zcomp_create(NAME) /* NAME e.g. "lzo" */
    
    which initialises compressing backend if requested algorithm is supported.
    
    Compress:
    	zstrm = zcomp_strm_find(comp)
    	zcomp_compress(comp, zstrm, src, &dst_len)
    	[..] /* copy compressed data */
    	zcomp_strm_release(comp, zstrm)
    
    Decompress:
    	zcomp_decompress(comp, src, src_len, dst);
    
    Free compessing backend and its zcomp stream:
    	zcomp_destroy(comp)
    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>
    e7e1ef43
zcomp.h 1.51 KB