• Huang Ying's avatar
    mm, swap: add swap readahead hit statistics · cbc65df2
    Huang Ying authored
    Patch series "mm, swap: VMA based swap readahead", v4.
    
    The swap readahead is an important mechanism to reduce the swap in
    latency.  Although pure sequential memory access pattern isn't very
    popular for anonymous memory, the space locality is still considered
    valid.
    
    In the original swap readahead implementation, the consecutive blocks in
    swap device are readahead based on the global space locality estimation.
    But the consecutive blocks in swap device just reflect the order of page
    reclaiming, don't necessarily reflect the access pattern in virtual
    memory space.  And the different tasks in the system may have different
    access patterns, which makes the global space locality estimation
    incorrect.
    
    In this patchset, when page fault occurs, the virtual pages near the
    fault address will be readahead instead of the swap slots near the fault
    swap slot in swap device.  This avoid to readahead the unrelated swap
    slots.  At the same time, the swap readahead is changed to work on
    per-VMA from globally.  So that the different access patterns of the
    different VMAs could be distinguished, and the different readahead
    policy could be applied accordingly.  The original core readahead
    detection and scaling algorithm is reused, because it is an effect
    algorithm to detect the space locality.
    
    In addition to the swap readahead changes, some new sysfs interface is
    added to show the efficiency of the readahead algorithm and some other
    swap statistics.
    
    This new implementation will incur more small random read, on SSD, the
    improved correctness of estimation and readahead target should beat the
    potential increased overhead, this is also illustrated in the test
    results below.  But on HDD, the overhead may beat the benefit, so the
    original implementation will be used by default.
    
    The test and result is as follow,
    
    Common test condition
    =====================
    
    Test Machine: Xeon E5 v3 (2 sockets, 72 threads, 32G RAM)
    Swap device: NVMe disk
    
    Micro-benchmark with combined access pattern
    ============================================
    
    vm-scalability, sequential swap test case, 4 processes to eat 50G
    virtual memory space, repeat the sequential memory writing until 300
    seconds.  The first round writing will trigger swap out, the following
    rounds will trigger sequential swap in and out.
    
    At the same time, run vm-scalability random swap test case in
    background, 8 processes to eat 30G virtual memory space, repeat the
    random memory write until 300 seconds.  This will trigger random swap-in
    in the background.
    
    This is a combined workload with sequential and random memory accessing
    at the same time.  The result (for sequential workload) is as follow,
    
    			Base		Optimized
    			----		---------
    throughput		345413 KB/s	414029 KB/s (+19.9%)
    latency.average		97.14 us	61.06 us (-37.1%)
    latency.50th		2 us		1 us
    latency.60th		2 us		1 us
    latency.70th		98 us		2 us
    latency.80th		160 us		2 us
    latency.90th		260 us		217 us
    latency.95th		346 us		369 us
    latency.99th		1.34 ms		1.09 ms
    ra_hit%			52.69%		99.98%
    
    The original swap readahead algorithm is confused by the background
    random access workload, so readahead hit rate is lower.  The VMA-base
    readahead algorithm works much better.
    
    Linpack
    =======
    
    The test memory size is bigger than RAM to trigger swapping.
    
    			Base		Optimized
    			----		---------
    elapsed_time		393.49 s	329.88 s (-16.2%)
    ra_hit%			86.21%		98.82%
    
    The score of base and optimized kernel hasn't visible changes.  But the
    elapsed time reduced and readahead hit rate improved, so the optimized
    kernel runs better for startup and tear down stages.  And the absolute
    value of readahead hit rate is high, shows that the space locality is
    still valid in some practical workloads.
    
    This patch (of 5):
    
    The statistics for total readahead pages and total readahead hits are
    recorded and exported via the following sysfs interface.
    
    /sys/kernel/mm/swap/ra_hits
    /sys/kernel/mm/swap/ra_total
    
    With them, the efficiency of the swap readahead could be measured, so
    that the swap readahead algorithm and parameters could be tuned
    accordingly.
    
    [akpm@linux-foundation.org: don't display swap stats if CONFIG_SWAP=n]
    Link: http://lkml.kernel.org/r/20170807054038.1843-2-ying.huang@intel.comSigned-off-by: default avatar"Huang, Ying" <ying.huang@intel.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Shaohua Li <shli@kernel.org>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Fengguang Wu <fengguang.wu@intel.com>
    Cc: Tim Chen <tim.c.chen@intel.com>
    Cc: Dave Hansen <dave.hansen@intel.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    cbc65df2
vmstat.c 47.7 KB