RPC Latency Profile

RAMCloud Benchmarks

A single backup operation (ClusterPerf with 100-byte writes, 1 master, 3 backups). For each operation, a timeline of events was logged. Not all timelines had the same "shape", as not all backup operations are handled by the same sequence of events. Thus, the most common timeline "shape" was chosen, and the details below present statistics about the most common timeline shape. This procedure was done for both the backup and the master. Based on source tree at commit 8f30e4e6d84fc2a13578ac0c92c84065047c0581.

On the Master

Averaged over 1912 same-shape timelines.

Timeline on a Master

0 us    --- Begin backup (BackupManager::sync())
| 
|
1.8 us  --- First write RPC sent out
| 
|
3.2 us  --- Second write RPC sent out
| 
|
4.4 us  --- Third write RPC sent out
| 
|
| [~ 4 us "dead time"]
|
|
8.2 us  --- First write RPC completes (duration: 6.4 us)
|
9.8 us  --- Second write RPC completes (duration: 6.6 us)
|
10.7 us --- Third write RPC completes (duration: 6.3 us)
10.8 us --- End backup

Major time sinks in issue path

Acquiring Dispatch::Lock in TransportManager::WorkerSession::clientSend for every write RPC
- Cost: 3 x ~300ns
InfRcTransport<Infiniband>::getTransmitBuffer(): waiting for free tx buffer for every write RPC
- Cost: 3 x ~200ns (first write RPC more expensive than 2nd and 3rd)
Calling into Infiniband transport: postSendZeroCopy (unavoidable?)
- Cost: 3 x ~400ns (first write RPC more expensive than 2nd and 3rd)

More detailed timeline

Below is a more detailed version of the timeline above which presents information about each and every recorded event in the code path:

More detailed timeline on a master

(         Cumulative          )  |               Delta                |  [ Std. Dev.  ] Event Description
(    Min      Mean      Max   )  |    Min       Mean          Max     |  [            ]
(                             )  |                                    |  [            ]
(    0.0 ns   0.0 ns   0.0 ns )  |    0.0 ns      0.0 ns      0.0 ns  |  [    0.0 ns  ] Invoking  sync() to backups
(   90.0 ns 139.0 ns 531.5 ns )  |   90.0 ns    139.0 ns    531.5 ns  |  [   35.0 ns  ] Invoking  Send 'write       segment' RPC
(  202.5 ns 280.2 ns 783.4 ns )  |  112.5 ns    141.1 ns    251.9 ns  |  [   12.7 ns  ] Invoking  client.send()
(  218.5 ns 296.7 ns 802.8 ns )  |   16.0 ns     16.5 ns     19.4 ns  |  [    0.8 ns  ] Invoking  session->clientSend()
(  441.4 ns 605.4 ns   5.4 us )  |  222.9 ns    308.8 ns      4.6 us  |  [  119.6 ns  ] Acquired Dispatch::Lock in TransportManager::WorkerSession::clientSend
(  497.0 ns 708.3 ns   5.6 us )  |   55.6 ns    102.8 ns    183.4 ns  |  [   16.1 ns  ] Invoking  Instantiate new ClientRpc
(  566.9 ns 841.6 ns  16.9 us )  |   69.9 ns    133.4 ns     11.3 us  |  [  256.0 ns  ] Completed Instantiate new ClientRpc
(  583.9 ns 862.9 ns  16.9 us )  |   17.0 ns     21.3 ns     45.0 ns  |  [    2.3 ns  ] Invoking  rpc->sendOrQueue
(  891.4 ns   1.3 us  21.4 us )  |  307.5 ns    387.9 ns      4.5 us  |  [   97.0 ns  ] Invoking  postSendZeroCopy
(    1.3 us   1.8 us  22.6 us )  |  455.4 ns    587.0 ns      1.2 us  |  [   47.2 ns  ] Completed postSendZeroCopy
(    1.5 us   2.0 us  31.6 us )  |  127.5 ns    196.4 ns      9.0 us  |  [  202.4 ns  ] Completed rpc->sendOrQueue
(    1.5 us   2.1 us  31.7 us )  |   54.5 ns     73.2 ns     94.4 ns  |  [    2.8 ns  ] Completed session->clientSend()
(    1.5 us   2.1 us  31.7 us )  |   16.0 ns     16.9 ns     20.5 ns  |  [    0.8 ns  ] Completed client.send()
(    1.6 us   2.1 us  31.8 us )  |   16.0 ns     21.8 ns     35.5 ns  |  [    2.3 ns  ] Completed Send 'write       segment' RPC
(    1.6 us   2.2 us  32.0 us )  |   37.5 ns     50.6 ns    224.0 ns  |  [   15.5 ns  ] Invoking  Send 'write       segment' RPC
(    1.7 us   2.3 us  32.2 us )  |   75.0 ns     98.3 ns    148.0 ns  |  [    9.2 ns  ] Invoking  client.send()
(    1.7 us   2.3 us  32.2 us )  |   16.0 ns     16.6 ns     19.4 ns  |  [    0.8 ns  ] Invoking  session->clientSend()
(    1.9 us   2.6 us  35.7 us )  |  175.9 ns    307.6 ns      3.5 us  |  [  143.2 ns  ] Acquired Dispatch::Lock in TransportManager::WorkerSession::clientSend
(    1.9 us   2.7 us  35.7 us )  |   33.4 ns     45.3 ns     70.9 ns  |  [    1.3 ns  ] Invoking  Instantiate new ClientRpc
(    1.9 us   2.7 us  35.8 us )  |   45.0 ns     49.1 ns     78.4 ns  |  [    4.3 ns  ] Completed Instantiate new ClientRpc
(    2.0 us   2.7 us  35.8 us )  |   16.0 ns     16.6 ns     20.5 ns  |  [    0.8 ns  ] Invoking  rpc->sendOrQueue
(    2.1 us   2.8 us  36.1 us )  |   95.1 ns    114.6 ns    270.0 ns  |  [    9.4 ns  ] Invoking  postSendZeroCopy
(    2.3 us   3.2 us  40.9 us )  |  288.1 ns    334.2 ns      4.8 us  |  [  104.3 ns  ] Completed postSendZeroCopy
(    2.4 us   3.2 us  41.0 us )  |   53.5 ns     58.3 ns    107.4 ns  |  [    4.6 ns  ] Completed rpc->sendOrQueue
(    2.4 us   3.3 us  41.1 us )  |   52.5 ns     71.7 ns     90.0 ns  |  [    3.0 ns  ] Completed session->clientSend()
(    2.5 us   3.3 us  41.1 us )  |   16.0 ns     16.3 ns     30.0 ns  |  [    0.6 ns  ] Completed client.send()
(    2.5 us   3.3 us  41.2 us )  |   16.0 ns     22.5 ns     64.1 ns  |  [    3.7 ns  ] Completed Send 'write       segment' RPC
(    2.5 us   3.4 us  41.3 us )  |   29.0 ns     44.6 ns     69.5 ns  |  [    6.5 ns  ] Invoking  Send 'write       segment' RPC
(    2.6 us   3.5 us  41.4 us )  |   69.5 ns     88.6 ns    148.0 ns  |  [   10.8 ns  ] Invoking  client.send()
(    2.6 us   3.5 us  41.5 us )  |   16.0 ns     19.2 ns     35.5 ns  |  [    3.3 ns  ] Invoking  session->clientSend()
(    2.8 us   3.8 us  45.5 us )  |  167.0 ns    302.3 ns      4.0 us  |  [  177.7 ns  ] Acquired Dispatch::Lock in TransportManager::WorkerSession::clientSend
(    2.8 us   3.8 us  45.5 us )  |   21.5 ns     30.0 ns     68.5 ns  |  [    5.0 ns  ] Invoking  Instantiate new ClientRpc
(    2.8 us   3.9 us  45.6 us )  |   42.6 ns     52.9 ns    109.4 ns  |  [    6.1 ns  ] Completed Instantiate new ClientRpc
(    2.8 us   3.9 us  45.7 us )  |   16.0 ns     16.5 ns     62.0 ns  |  [    1.3 ns  ] Invoking  rpc->sendOrQueue
(    3.0 us   4.0 us  46.0 us )  |  119.0 ns    146.3 ns    259.4 ns  |  [    9.2 ns  ] Invoking  postSendZeroCopy
(    3.3 us   4.4 us  50.9 us )  |  311.9 ns    368.3 ns      4.9 us  |  [  147.6 ns  ] Completed postSendZeroCopy
(    3.3 us   4.5 us  59.8 us )  |   53.5 ns     60.1 ns      8.9 us  |  [  202.2 ns  ] Completed rpc->sendOrQueue
(    3.4 us   4.5 us  59.9 us )  |   52.5 ns     56.5 ns    101.9 ns  |  [    4.9 ns  ] Completed session->clientSend()
(    3.4 us   4.5 us  59.9 us )  |   16.0 ns     16.7 ns     39.5 ns  |  [    1.0 ns  ] Completed client.send()
(    3.4 us   4.6 us  59.9 us )  |   16.0 ns     34.4 ns     43.0 ns  |  [    8.4 ns  ] Completed Send 'write       segment' RPC
(    4.0 us   8.2 us  68.4 us )  |  581.9 ns      3.6 us      8.5 us  |  [  714.2 ns  ] Reaped outstanding RPC
(    4.3 us   9.8 us 116.3 us )  |  287.0 ns      1.6 us     47.9 us  |  [    1.4 us  ] Reaped outstanding RPC
(    4.6 us  10.7 us 121.5 us )  |  276.5 ns    959.9 ns      5.2 us  |  [  646.2 ns  ] Reaped outstanding RPC
(    4.6 us  10.8 us 126.1 us )  |   48.1 ns     60.2 ns      4.6 us  |  [  104.0 ns  ] Completed sync() to backups

Cumulative distributions for each event

Each event in the timeline is associated with a (cumulative) distribution of "completion" times. The "completion" time for a given event is the time delta between that event and its preceding event; hence, the cdf for the first event is quite boring. The major time sinks described above are labeled by their corresponding number. (Note: the x-axis in the cdf's below is in seconds.)

`Event Invoking sync() to backups`

master_event00.dat.png

`Event Invoking Send 'write segment' RPC`

`Event Invoking client.send()`

`Event Invoking session->clientSend()`

`Event Acquired Dispatch::Lock in TransportManager::WorkerSession::clientSend [1]`

`Event Invoking Instantiate new ClientRpc`

`Event Completed Instantiate new ClientRpc`

`Event Invoking rpc->sendOrQueue`

`Event Invoking postSendZeroCopy [2]`

`Event Completed postSendZeroCopy [3]`

`Event Completed rpc->sendOrQueue`

`Event Completed session->clientSend()`

`Event Completed client.send()`

`Event Completed Send 'write segment' RPC`

`Event Invoking Send 'write segment' RPC`

`Event Invoking client.send()`

`Event Invoking session->clientSend()`

`Event Acquired Dispatch::Lock in TransportManager::WorkerSession::clientSend [1]`

`Event Invoking Instantiate new ClientRpc`

`Event Completed Instantiate new ClientRpc`

`Event Invoking rpc->sendOrQueue`

`Event Invoking postSendZeroCopy [2]`

`Event Completed postSendZeroCopy [3]`

`Event Completed rpc->sendOrQueue`

`Event Completed session->clientSend()`

`Event Completed client.send()`

`Event Completed Send 'write segment' RPC`

`Event Invoking Send 'write segment' RPC`

`Event Invoking client.send()`

`Event Invoking session->clientSend()`

`Event Acquired Dispatch::Lock in TransportManager::WorkerSession::clientSend [1]`

`Event Invoking Instantiate new ClientRpc`

`Event Completed Instantiate new ClientRpc`

`Event Invoking rpc->sendOrQueue`

`Event Invoking postSendZeroCopy [2]`

`Event Completed postSendZeroCopy [3]`

`Event Completed rpc->sendOrQueue`

`Event Completed session->clientSend()`

`Event Completed client.send()`

`Event Completed Send 'write segment' RPC`

`Event Reaped outstanding RPC`

`Event Completed sync() to backups`

master_event43.dat.png

On the the Backup

Averaged over 3515 same-shape timelines.

Timeline on Backup

0 us   --- InfRcTransport Poller picks ups incoming RPC [dispatch thread]
|
325 ns --- Invoke service.handleRpc()                   [worker thread]
|
885 ns --- Completed service.handleRpc()                [worker thread]
|
1 us   --- Begin sending reply                          [dispatch thread]
|
|
|
1.8 us --- Completed sending reply                      [dispatch thread]

More detailed timeline

Just as we did for the master, we have a more detailed timeline for the events on the backup server:

More detailed timeline on a backup

(          Cumulative         )  |                Delta               |  [ Std. Dev.  ] Event Description 
(    Min      Mean     Max    )  |    Min         Mean        Max     |  [            ]
(                             )  |                                    |  [            ]
(    0.0 ns   0.0 ns   0.0 ns )  |    0.0 ns      0.0 ns      0.0 ns  |  [    0.0 ns  ] Invoking  append to buffer [dispatch]
(   34.4 ns  76.2 ns   2.8 us )  |   34.4 ns     76.2 ns      2.8 us  |  [   47.6 ns  ] Completed append to buffer [dispatch]
(   51.4 ns 102.1 ns   2.9 us )  |   17.0 ns     25.8 ns     56.9 ns  |  [    8.0 ns  ] Invoking  serviceManager->handleRpc() [dispatch]
(  146.5 ns 226.0 ns   5.8 us )  |   95.1 ns    124.0 ns      2.9 us  |  [  102.8 ns  ] Completed serviceManager->handleRpc() [dispatch]
(  170.0 ns 325.7 ns  68.3 us )  |   23.5 ns     99.7 ns     62.5 us  |  [    1.3 us  ] Invoking  service.handleRpc() [worker]
(  275.0 ns 458.8 ns  68.6 us )  |  105.0 ns    133.1 ns    374.7 ns  |  [   29.3 ns  ] Invoking  service.dispatch() [worker]
(  295.1 ns 484.3 ns  68.8 us )  |   20.1 ns     25.5 ns    212.1 ns  |  [    7.0 ns  ] Invoking  callHandler() [worker]
(  378.3 ns 589.2 ns  73.9 us )  |   83.2 ns    104.9 ns      5.1 us  |  [  107.2 ns  ] Invoking  SegmentInfo->write() [worker]
(  492.5 ns 774.0 ns  79.1 us )  |  114.2 ns    184.8 ns      5.2 us  |  [  115.6 ns  ] Completed SegmentInfo->write() [worker]
(  523.5 ns 823.0 ns  79.3 us )  |   31.0 ns     49.0 ns    116.9 ns  |  [    3.0 ns  ] Completed callHandler() [worker]
(  553.5 ns 854.3 ns  79.4 us )  |   30.0 ns     31.2 ns     90.0 ns  |  [    1.6 ns  ] Completed service.dispatch() [worker]
(  576.7 ns 885.0 ns  79.4 us )  |   23.2 ns     30.7 ns     93.1 ns  |  [    7.3 ns  ] Completed service.handleRpc() [worker]
(  675.9 ns   1.0 us  83.1 us )  |   99.2 ns    153.7 ns      3.7 us  |  [  109.4 ns  ] Invoking  worker->rpc->sendReply() [dispatch]
(  858.0 ns   1.3 us  86.2 us )  |  182.1 ns    224.8 ns      3.1 us  |  [   70.2 ns  ] Invoking  postSend() [dispatch]
(    1.1 us   1.6 us  90.7 us )  |  290.1 ns    330.6 ns      4.5 us  |  [  113.1 ns  ] Completed postSend() [dispatch]
(    1.3 us   1.8 us  93.8 us )  |  169.1 ns    216.5 ns      3.1 us  |  [   98.8 ns  ] Completed worker->rpc->sendReply() [dispatch]