Checkpoint expel stops before low mark

    const auto vbuckets = bucket.getVBuckets().getVBucketsSortedByChkMgrMem();

    for (const auto& it : vbuckets) {

        const auto vbid = it.first;

        VBucketPtr vb = bucket.getVBucket(vbid);

        if (!vb) {

            continue;

        }

        const auto expelResult =

                vb->checkpointManager->expelUnreferencedCheckpointItems();

        EP_LOG_DEBUG(

                "Expelled {} unreferenced checkpoint items "

                "from {} "

                "and estimated to have recovered {} bytes.",

                expelResult.count,

                vbid,

                expelResult.memory);

        if (bucket.getRequiredCheckpointMemoryReduction() == 0) {

            // All done

            return ReductionRequired::No;

        }

    }

size_t KVBucket::getRequiredCheckpointMemoryReduction() const {

    const auto checkpointMemoryRatio = getCheckpointMemoryRatio();

    const auto checkpointQuota = stats.getMaxDataSize() * checkpointMemoryRatio;

    const auto recoveryThreshold =

            checkpointQuota * getCheckpointMemoryRecoveryUpperMark();

    const auto usage = stats.getCheckpointManagerEstimatedMemUsage();

    if (usage < recoveryThreshold) {

        return 0;

    }

    const auto lowerRatio = getCheckpointMemoryRecoveryLowerMark();

    const auto lowerMark = checkpointQuota * lowerRatio;

    Expects(usage > lowerMark);

    const size_t amountOfMemoryToClear = usage - lowerMark;

    const auto toMB = [](size_t bytes) { return bytes / (1024 * 1024); };

    const auto upperRatio = getCheckpointMemoryRecoveryUpperMark();

    EP_LOG_DEBUG(

            "Triggering memory recovery as checkpoint memory usage ({} MB) "

            "exceeds the upper_mark ({}, "

            "{} MB) - total checkpoint quota {}, {} MB . Attempting to free {} "

            "MB of memory.",

            toMB(usage),

            upperRatio,

            toMB(checkpointQuota * upperRatio),

            checkpointMemoryRatio,

            toMB(checkpointQuota),

            toMB(amountOfMemoryToClear));

    return amountOfMemoryToClear;

}

If checkpoint memory usage exceeds high mark:

 -> amount of memory to recover to reach the low mark

else:

 -> 0

ClosedUnrefCheckpointRemoverTask:0 Triggering checkpoint memory recovery - attempting to free X MB