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    Subject[RFC PATCH 3/3] mm/zsmalloc: change ZS_MAX_PAGES_PER_ZSPAGE
    Date
    ZS_MAX_PAGES_PER_ZSPAGE does not have to be order or 2. The existing
    limit of 4 pages per zspage sets a tight limit on ->huge classes, which
    results in increased memory wastage and consumption.

    For example, on x86_64, PAGE_SHIFT 12, ->huge class_size range is

    ZS_MAX_PAGES_PER_ZSPAGE ->huge classes size range
    4 3280-4096
    5 3424-4096
    6 3520-4096

    With bigger ZS_MAX_PAGES_PER_ZSPAGE we have less ->huge classes, because
    some of the previously known as ->huge classes now have better chances to
    form zspages that will waste less memory. This increases the density and
    improves memory efficiency.

    Example,

    class_size 3328 with ZS_MAX_PAGES_PER_ZSPAGE=5 has pages_per_zspage 5
    and max_objects 6, while with ZS_MAX_PAGES_PER_ZSPAGE=1 it had
    pages_per_zspage 1 and max_objects 1. So now every 6th 3328-bytes object
    stored by zram will not consume a new zspage (and order-0 page), but will
    share an already allocated one.

    TEST
    ====

    Create a text file and do rounds of dd (one process). The amount of
    copied data, its content and order are stable.

    test script:

    rm /tmp/test-file
    for i in {1..200}; do
    cat /media/dev/linux-mmots/mm/zsmalloc.c >> /tmp/test-file;
    done

    for i in {1..5}; do
    umount /zram
    rmmod zram

    # create a 4G zram device, LZ0, multi stream, ext4 fs
    ./create-zram 4g

    for k in {1..3}; do
    j=1;
    while [ $j -lt $((1024*1024)) ]; do
    dd if=/tmp/test-file of=/zram/file-$k-$j bs=$j count=1 \
    oflag=sync > /dev/null 2>&1
    let j=$j+512
    done
    done

    sync
    cat /sys/block/zram0/mm_stat >> /tmp/zram-stat
    umount /zram
    rmmod zram
    done

    RESULTS
    =======
    cat /sys/block/zram0/mm_stat column 3 is zs_get_total_pages() << PAGE_SHIFT

    BASE
    3371106304 1714719722 1842778112 0 1842778112 16 0 1
    3371098112 1714667024 1842831360 0 1842831360 16 0 1
    3371110400 1714767329 1842716672 0 1842716672 16 0 1
    3371110400 1714717615 1842601984 0 1842601984 16 0 1
    3371106304 1714744207 1842135040 0 1842135040 16 0 1

    ZS_MAX_PAGES_PER_ZSPAGE=5
    3371094016 1714584459 1804095488 0 1804095488 16 0 1
    3371102208 1714619140 1804660736 0 1804660736 16 0 1
    3371114496 1714755452 1804316672 0 1804316672 16 0 1
    3371081728 1714606179 1804800000 0 1804800000 16 0 1
    3371122688 1714871507 1804361728 0 1804361728 16 0 1

    ZS_MAX_PAGES_PER_ZSPAGE=6
    3371114496 1714704275 1789206528 0 1789206528 16 0 1
    3371102208 1714740225 1789259776 0 1789259776 16 0 1
    3371102208 1714717465 1789071360 0 1789071360 16 0 1
    3371110400 1714704079 1789194240 0 1789194240 16 0 1
    3371085824 1714792954 1789308928 0 1789308928 16 0 1

    So that's
    around 36MB of saved space between BASE and ZS_MAX_PAGES_PER_ZSPAGE=5
    and
    around 51MB of saved space between BASE and ZS_MAX_PAGES_PER_ZSPAGE=6.

    Set ZS_MAX_PAGES_PER_ZSPAGE to 6 for now.

    Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
    ---
    mm/zsmalloc.c | 7 +++----
    1 file changed, 3 insertions(+), 4 deletions(-)

    diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
    index 61b1b35..0c9f117 100644
    --- a/mm/zsmalloc.c
    +++ b/mm/zsmalloc.c
    @@ -74,11 +74,10 @@
    #define ZS_ALIGN 8

    /*
    - * A single 'zspage' is composed of up to 2^N discontiguous 0-order (single)
    - * pages. ZS_MAX_ZSPAGE_ORDER defines upper limit on N.
    + * A single 'zspage' is composed of up ZS_MAX_PAGES_PER_ZSPAGE discontiguous
    + * 0-order (single) pages.
    */
    -#define ZS_MAX_ZSPAGE_ORDER 2
    -#define ZS_MAX_PAGES_PER_ZSPAGE (_AC(1, UL) << ZS_MAX_ZSPAGE_ORDER)
    +#define ZS_MAX_PAGES_PER_ZSPAGE 6

    #define ZS_HANDLE_SIZE (sizeof(unsigned long))

    --
    2.7.1
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