Uploaded image for project: 'Couchbase Server'
  1. Couchbase Server
  2. MB-43055

ItemPager available flag not reset if run() early exits

    XMLWordPrintable

    Details

    • Triage:
      Triaged
    • Story Points:
      1
    • Is this a Regression?:
      Yes
    • Sprint:
      KV-Engine 2021-Jan

      Description

      There is a bug which, if triggered, will stop a bucket from successfully evicting ever again (until restart/bucket deletion).


      Bug introduced in http://review.couchbase.org/c/kv_engine/+/133197 (6.6.1 pre-GA).

      item_pager.cc,ItemPager::run

      112
          if (((current > upper) || doEvict || wasNotified) &&
      113
              (*available).compare_exchange_strong(inverse, false)) {
      114
              if (kvBucket->getItemEvictionPolicy() == EvictionPolicy::Value) {
      115
                  doEvict = true;
      116
              }
      117
       
      118
              ++stats.pagerRuns;
      119
       
      120
              if (current <= lower) {
      121
                  // early exit - no need to run a paging visitor
      122
                  return true;
      123
              }
      

      The above change introduced the highlighted early exit; in the event that the item pager is notified to run, but the memory usage has dropped below the low watermark, don't evict. However, this is not safe to do -

      item_pager.cc,ItemPager::run

      112
          if (((current > upper) || doEvict || wasNotified) &&
      113
              (*available).compare_exchange_strong(inverse, false)) {
      

      Available is an atomic bool used to track if an item pager is currently running. It has been set to false here, but is only reset once the created PagingVisitor is completed. If the visitor creation is skipped, the flag is never reset, so no further item pager runs will occur.

        Attachments

          Issue Links

          No reviews matched the request. Check your Options in the drop-down menu of this sections header.

            Activity

            There are no commits yet on this issue.

              People

              Assignee:
              ashwin.govindarajulu Ashwin Govindarajulu
              Reporter:
              james.harrison James Harrison
              Votes:
              0 Vote for this issue
              Watchers:
              6 Start watching this issue

                Dates

                Created:
                Updated:
                Resolved:

                  PagerDuty