1. 28 Mar, 2017 13 commits
    • Shaohua Li's avatar
      blk-throttle: add a simple idle detection · 9e234eea
      Shaohua Li authored
      A cgroup gets assigned a low limit, but the cgroup could never dispatch
      enough IO to cross the low limit. In such case, the queue state machine
      will remain in LIMIT_LOW state and all other cgroups will be throttled
      according to low limit. This is unfair for other cgroups. We should
      treat the cgroup idle and upgrade the state machine to lower state.
      
      We also have a downgrade logic. If the state machine upgrades because of
      cgroup idle (real idle), the state machine will downgrade soon as the
      cgroup is below its low limit. This isn't what we want. A more
      complicated case is cgroup isn't idle when queue is in LIMIT_LOW. But
      when queue gets upgraded to lower state, other cgroups could dispatch
      more IO and this cgroup can't dispatch enough IO, so the cgroup is below
      its low limit and looks like idle (fake idle). In this case, the queue
      should downgrade soon. The key to determine if we should do downgrade is
      to detect if cgroup is truely idle.
      
      Unfortunately it's very hard to determine if a cgroup is real idle. This
      patch uses the 'think time check' idea from CFQ for the purpose. Please
      note, the idea doesn't work for all workloads. For example, a workload
      with io depth 8 has disk utilization 100%, hence think time is 0, eg,
      not idle. But the workload can run higher bandwidth with io depth 16.
      Compared to io depth 16, the io depth 8 workload is idle. We use the
      idea to roughly determine if a cgroup is idle.
      
      We treat a cgroup idle if its think time is above a threshold (by
      default 1ms for SSD and 100ms for HD). The idea is think time above the
      threshold will start to harm performance. HD is much slower so a longer
      think time is ok.
      
      The patch (and the latter patches) uses 'unsigned long' to track time.
      We convert 'ns' to 'us' with 'ns >> 10'. This is fast but loses
      precision, should not a big deal.
      Signed-off-by: default avatarShaohua Li <shli@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      9e234eea
    • Shaohua Li's avatar
      blk-throttle: make bandwidth change smooth · 7394e31f
      Shaohua Li authored
      When cgroups all reach low limit, cgroups can dispatch more IO. This
      could make some cgroups dispatch more IO but others not, and even some
      cgroups could dispatch less IO than their low limit. For example, cg1
      low limit 10MB/s, cg2 limit 80MB/s, assume disk maximum bandwidth is
      120M/s for the workload. Their bps could something like this:
      
      cg1/cg2 bps: T1: 10/80 -> T2: 60/60 -> T3: 10/80
      
      At T1, all cgroups reach low limit, so they can dispatch more IO later.
      Then cg1 dispatch more IO and cg2 has no room to dispatch enough IO. At
      T2, cg2 only dispatches 60M/s. Since We detect cg2 dispatches less IO
      than its low limit 80M/s, we downgrade the queue from LIMIT_MAX to
      LIMIT_LOW, then all cgroups are throttled to their low limit (T3). cg2
      will have bandwidth below its low limit at most time.
      
      The big problem here is we don't know the maximum bandwidth of the
      workload, so we can't make smart decision to avoid the situation. This
      patch makes cgroup bandwidth change smooth. After disk upgrades from
      LIMIT_LOW to LIMIT_MAX, we don't allow cgroups use all bandwidth upto
      their max limit immediately. Their bandwidth limit will be increased
      gradually to avoid above situation. So above example will became
      something like:
      
      cg1/cg2 bps: 10/80 -> 15/105 -> 20/100 -> 25/95 -> 30/90 -> 35/85 -> 40/80
      -> 45/75 -> 22/98
      
      In this way cgroups bandwidth will be above their limit in majority
      time, this still doesn't fully utilize disk bandwidth, but that's
      something we pay for sharing.
      
      Scale up is linear. The limit scales up 1/2 .low limit every
      throtl_slice after upgrade. The scale up will stop if the adjusted limit
      hits .max limit. Scale down is exponential. We cut the scale value half
      if a cgroup doesn't hit its .low limit. If the scale becomes 0, we then
      fully downgrade the queue to LIMIT_LOW state.
      
      Note this doesn't completely avoid cgroup running under its low limit.
      The best way to guarantee cgroup doesn't run under its limit is to set
      max limit. For example, if we set cg1 max limit to 40, cg2 will never
      run under its low limit.
      Signed-off-by: default avatarShaohua Li <shli@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      7394e31f
    • Shaohua Li's avatar
      blk-throttle: detect completed idle cgroup · aec24246
      Shaohua Li authored
      cgroup could be assigned a limit, but doesn't dispatch enough IO, eg the
      cgroup is idle. When this happens, the cgroup doesn't hit its limit, so
      we can't move the state machine to higher level and all cgroups will be
      throttled to their lower limit, so we waste bandwidth. Detecting idle
      cgroup is hard. This patch handles a simple case, a cgroup doesn't
      dispatch any IO. We ignore such cgroup's limit, so other cgroups can use
      the bandwidth.
      
      Please note this will be replaced with a more sophisticated algorithm
      later, but this demonstrates the idea how we handle idle cgroups, so I
      leave it here.
      Signed-off-by: default avatarShaohua Li <shli@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      aec24246
    • Shaohua Li's avatar
      blk-throttle: choose a small throtl_slice for SSD · d61fcfa4
      Shaohua Li authored
      The throtl_slice is 100ms by default. This is a long time for SSD, a lot
      of IO can run. To make cgroups have smoother throughput, we choose a
      small value (20ms) for SSD.
      Signed-off-by: default avatarShaohua Li <shli@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      d61fcfa4
    • Shaohua Li's avatar
      blk-throttle: make throtl_slice tunable · 297e3d85
      Shaohua Li authored
      throtl_slice is important for blk-throttling. It's called slice
      internally but it really is a time window blk-throttling samples data.
      blk-throttling will make decision based on the samplings. An example is
      bandwidth measurement. A cgroup's bandwidth is measured in the time
      interval of throtl_slice.
      
      A small throtl_slice meanse cgroups have smoother throughput but burn
      more CPUs. It has 100ms default value, which is not appropriate for all
      disks. A fast SSD can dispatch a lot of IOs in 100ms. This patch makes
      it tunable.
      
      Since throtl_slice isn't a time slice, the sysfs name
      'throttle_sample_time' reflects its character better.
      Signed-off-by: default avatarShaohua Li <shli@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      297e3d85
    • Shaohua Li's avatar
      blk-throttle: make sure expire time isn't too big · 06cceedc
      Shaohua Li authored
      cgroup could be throttled to a limit but when all cgroups cross high
      limit, queue enters a higher state and so the group should be throttled
      to a higher limit. It's possible the cgroup is sleeping because of
      throttle and other cgroups don't dispatch IO any more. In this case,
      nobody can trigger current downgrade/upgrade logic. To fix this issue,
      we could either set up a timer to wakeup the cgroup if other cgroups are
      idle or make sure this cgroup doesn't sleep too long. Setting up a timer
      means we must change the timer very frequently. This patch chooses the
      latter. Making cgroup sleep time not too big wouldn't change cgroup
      bps/iops, but could make it wakeup more frequently, which isn't a big
      issue because throtl_slice * 8 is already quite big.
      Signed-off-by: default avatarShaohua Li <shli@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      06cceedc
    • Shaohua Li's avatar
      blk-throttle: add downgrade logic · 3f0abd80
      Shaohua Li authored
      When queue state machine is in LIMIT_MAX state, but a cgroup is below
      its low limit for some time, the queue should be downgraded to lower
      state as one cgroup's low limit isn't met.
      Signed-off-by: default avatarShaohua Li <shli@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      3f0abd80
    • Shaohua Li's avatar
      blk-throttle: add upgrade logic for LIMIT_LOW state · c79892c5
      Shaohua Li authored
      When queue is in LIMIT_LOW state and all cgroups with low limit cross
      the bps/iops limitation, we will upgrade queue's state to
      LIMIT_MAX. To determine if a cgroup exceeds its limitation, we check if
      the cgroup has pending request. Since cgroup is throttled according to
      the limit, pending request means the cgroup reaches the limit.
      
      If a cgroup has limit set for both read and write, we consider the
      combination of them for upgrade. The reason is read IO and write IO can
      interfere with each other. If we do the upgrade based in one direction
      IO, the other direction IO could be severly harmed.
      
      For a cgroup hierarchy, there are two cases. Children has lower low
      limit than parent. Parent's low limit is meaningless. If children's
      bps/iops cross low limit, we can upgrade queue state. The other case is
      children has higher low limit than parent. Children's low limit is
      meaningless. As long as parent's bps/iops (which is a sum of childrens
      bps/iops) cross low limit, we can upgrade queue state.
      Signed-off-by: default avatarShaohua Li <shli@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      c79892c5
    • Shaohua Li's avatar
      blk-throttle: configure bps/iops limit for cgroup in low limit · b22c417c
      Shaohua Li authored
      each queue will have a state machine. Initially queue is in LIMIT_LOW
      state, which means all cgroups will be throttled according to their low
      limit. After all cgroups with low limit cross the limit, the queue state
      gets upgraded to LIMIT_MAX state.
      For max limit, cgroup will use the limit configured by user.
      For low limit, cgroup will use the minimal value between low limit and
      max limit configured by user. If the minimal value is 0, which means the
      cgroup doesn't configure low limit, we will use max limit to throttle
      the cgroup and the cgroup is ready to upgrade to LIMIT_MAX
      Signed-off-by: default avatarShaohua Li <shli@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      b22c417c
    • Shaohua Li's avatar
      blk-throttle: add .low interface · cd5ab1b0
      Shaohua Li authored
      Add low limit for cgroup and corresponding cgroup interface. To be
      consistent with memcg, we allow users configure .low limit higher than
      .max limit. But the internal logic always assumes .low limit is lower
      than .max limit. So we add extra bps/iops_conf fields in throtl_grp for
      userspace configuration. Old bps/iops fields in throtl_grp will be the
      actual limit we use for throttling.
      Signed-off-by: default avatarShaohua Li <shli@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      cd5ab1b0
    • Shaohua Li's avatar
      blk-throttle: add configure option for new .low interface · 327ffb9b
      Shaohua Li authored
      As discussed in LSF, add configure option for the interface and mark it
      as experimental, so people can try/test.
      Signed-off-by: default avatarShaohua Li <shli@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      327ffb9b
    • Shaohua Li's avatar
      blk-throttle: prepare support multiple limits · 9f626e37
      Shaohua Li authored
      We are going to support low/max limit, each cgroup will have 2 limits
      after that. This patch prepares for the multiple limits change.
      Signed-off-by: default avatarShaohua Li <shli@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      9f626e37
    • Shaohua Li's avatar
      blk-throttle: use U64_MAX/UINT_MAX to replace -1 · 2ab5492d
      Shaohua Li authored
      clean up the code to avoid using -1
      Signed-off-by: default avatarShaohua Li <shli@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      2ab5492d
  2. 25 Mar, 2017 1 commit
  3. 24 Mar, 2017 4 commits
  4. 23 Mar, 2017 16 commits
  5. 21 Mar, 2017 6 commits
    • Jens Axboe's avatar
      block: fix stacked driver stats init and free · a83b576c
      Jens Axboe authored
      If a driver allocates a queue for stacked usage, then it does
      not currently get stats allocated. This causes the later init
      of, eg, writeback throttling to blow up. Move the init to the
      queue allocation instead.
      
      Additionally, allow a NULL callback unregistration. This avoids
      having the caller check for that, fixing another oops on
      removal of a block device that doesn't have poll stats allocated.
      
      Fixes: 34dbad5d ("blk-stat: convert to callback-based statistics reporting")
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      a83b576c
    • Omar Sandoval's avatar
      blk-stat: convert to callback-based statistics reporting · 34dbad5d
      Omar Sandoval authored
      Currently, statistics are gathered in ~0.13s windows, and users grab the
      statistics whenever they need them. This is not ideal for both in-tree
      users:
      
      1. Writeback throttling wants its own dynamically sized window of
         statistics. Since the blk-stats statistics are reset after every
         window and the wbt windows don't line up with the blk-stats windows,
         wbt doesn't see every I/O.
      2. Polling currently grabs the statistics on every I/O. Again, depending
         on how the window lines up, we may miss some I/Os. It's also
         unnecessary overhead to get the statistics on every I/O; the hybrid
         polling heuristic would be just as happy with the statistics from the
         previous full window.
      
      This reworks the blk-stats infrastructure to be callback-based: users
      register a callback that they want called at a given time with all of
      the statistics from the window during which the callback was active.
      Users can dynamically bucketize the statistics. wbt and polling both
      currently use read vs. write, but polling can be extended to further
      subdivide based on request size.
      
      The callbacks are kept on an RCU list, and each callback has percpu
      stats buffers. There will only be a few users, so the overhead on the
      I/O completion side is low. The stats flushing is also simplified
      considerably: since the timer function is responsible for clearing the
      statistics, we don't have to worry about stale statistics.
      
      wbt is a trivial conversion. After the conversion, the windowing problem
      mentioned above is fixed.
      
      For polling, we register an extra callback that caches the previous
      window's statistics in the struct request_queue for the hybrid polling
      heuristic to use.
      
      Since we no longer have a single stats buffer for the request queue,
      this also removes the sysfs and debugfs stats entries. To replace those,
      we add a debugfs entry for the poll statistics.
      Signed-off-by: default avatarOmar Sandoval <osandov@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      34dbad5d
    • Omar Sandoval's avatar
      blk-stat: move BLK_RQ_STAT_BATCH definition to blk-stat.c · 4875253f
      Omar Sandoval authored
      This is an implementation detail that no-one outside of blk-stat.c uses.
      Signed-off-by: default avatarOmar Sandoval <osandov@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      4875253f
    • Omar Sandoval's avatar
      blk-stat: use READ and WRITE instead of BLK_STAT_{READ,WRITE} · fa2e39cb
      Omar Sandoval authored
      The stats buckets will become generic soon, so make the existing users
      use the common READ and WRITE definitions instead of one internal to
      blk-stat.
      Signed-off-by: default avatarOmar Sandoval <osandov@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      fa2e39cb
    • Omar Sandoval's avatar
      block: remove extra calls to wbt_exit() · 0315b159
      Omar Sandoval authored
      We always call wbt_exit() from blk_release_queue(), so these are
      unnecessary.
      Signed-off-by: default avatarOmar Sandoval <osandov@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      0315b159
    • Omar Sandoval's avatar
      blk-stat: fix blk_stat_sum() if all samples are batched · 7d8d0014
      Omar Sandoval authored
      We need to flush the batch _before_ we check the number of samples,
      otherwise we'll miss all of the batched samples.
      
      Fixes: cf43e6be ("block: add scalable completion tracking of requests")
      Signed-off-by: default avatarOmar Sandoval <osandov@fb.com>
      Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      7d8d0014