Eric Lee / smarc-fsl-linux-kernel

27 Jul, 2016

3 commits

16 Jun, 2016

1 commit

  • 35c55c987   tipc: add neighbor monitoring framework ... Browse Code »

    TIPC based clusters are by default set up with full-mesh link
    connectivity between all nodes. Those links are expected to provide
    a short failure detection time, by default set to 1500 ms. Because
    of this, the background load for neighbor monitoring in an N-node
    cluster increases with a factor N on each node, while the overall
    monitoring traffic through the network infrastructure increases at
    a ~(N * (N - 1)) rate. Experience has shown that such clusters don't
    scale well beyond ~100 nodes unless we significantly increase failure
    discovery tolerance.

    This commit introduces a framework and an algorithm that drastically
    reduces this background load, while basically maintaining the original
    failure detection times across the whole cluster. Using this algorithm,
    background load will now grow at a rate of ~(2 * sqrt(N)) per node, and
    at ~(2 * N * sqrt(N)) in traffic overhead. As an example, each node will
    now have to actively monitor 38 neighbors in a 400-node cluster, instead
    of as before 399.

    This "Overlapping Ring Supervision Algorithm" is completely distributed
    and employs no centralized or coordinated state. It goes as follows:

    - Each node makes up a linearly ascending, circular list of all its N
    known neighbors, based on their TIPC node identity. This algorithm
    must be the same on all nodes.

    - The node then selects the next M = sqrt(N) - 1 nodes downstream from
    itself in the list, and chooses to actively monitor those. This is
    called its "local monitoring domain".

    - It creates a domain record describing the monitoring domain, and
    piggy-backs this in the data area of all neighbor monitoring messages
    (LINK_PROTOCOL/STATE) leaving that node. This means that all nodes in
    the cluster eventually (default within 400 ms) will learn about
    its monitoring domain.

    - Whenever a node discovers a change in its local domain, e.g., a node
    has been added or has gone down, it creates and sends out a new
    version of its node record to inform all neighbors about the change.

    - A node receiving a domain record from anybody outside its local domain
    matches this against its own list (which may not look the same), and
    chooses to not actively monitor those members of the received domain
    record that are also present in its own list. Instead, it relies on
    indications from the direct monitoring nodes if an indirectly
    monitored node has gone up or down. If a node is indicated lost, the
    receiving node temporarily activates its own direct monitoring towards
    that node in order to confirm, or not, that it is actually gone.

    - Since each node is actively monitoring sqrt(N) downstream neighbors,
    each node is also actively monitored by the same number of upstream
    neighbors. This means that all non-direct monitoring nodes normally
    will receive sqrt(N) indications that a node is gone.

    - A major drawback with ring monitoring is how it handles failures that
    cause massive network partitionings. If both a lost node and all its
    direct monitoring neighbors are inside the lost partition, the nodes in
    the remaining partition will never receive indications about the loss.
    To overcome this, each node also chooses to actively monitor some
    nodes outside its local domain. Those nodes are called remote domain
    "heads", and are selected in such a way that no node in the cluster
    will be more than two direct monitoring hops away. Because of this,
    each node, apart from monitoring the member of its local domain, will
    also typically monitor sqrt(N) remote head nodes.

    - As an optimization, local list status, domain status and domain
    records are marked with a generation number. This saves senders from
    unnecessarily conveying unaltered domain records, and receivers from
    performing unneeded re-adaptations of their node monitoring list, such
    as re-assigning domain heads.

    - As a measure of caution we have added the possibility to disable the
    new algorithm through configuration. We do this by keeping a threshold
    value for the cluster size; a cluster that grows beyond this value
    will switch from full-mesh to ring monitoring, and vice versa when
    it shrinks below the value. This means that if the threshold is set to
    a value larger than any anticipated cluster size (default size is 32)
    the new algorithm is effectively disabled. A patch set for altering the
    threshold value and for listing the table contents will follow shortly.

    - This change is fully backwards compatible.

    Acked-by: Ying Xue
    Signed-off-by: Jon Maloy
    Signed-off-by: David S. Miller

    Jon Paul Maloy