Watch Memory Usage Benchmark

NOTE: The watch features are under active development, and their memory usage may change as that development progresses. We do not expect it to significantly increase beyond the figures stated below.

A primary goal of etcd is supporting a very large number of watchers doing a massively large amount of watching. etcd aims to support O(10k) clients, O(100K) watch streams (O(10) streams per client) and O(10M) total watchings (O(100) watching per stream). The memory consumed by each individual watching accounts for the largest portion of etcd’s overall usage, and is therefore the focus of current and future optimizations.

Three related components of etcd watch consume physical memory: each grpc.Conn, each watch stream, and each instance of the watching activity. grpc.Conn maintains the actual TCP connection and other gRPC connection state. Each grpc.Conn consumes O(10kb) of memory, and might have multiple watch streams attached.

Each watch stream is an independent HTTP2 connection which consumes another O(10kb) of memory. Multiple watchings might share one watch stream.

Watching is the actual struct that tracks the changes on the key-value store. Each watching should only consume < O(1kb).

                                          +-------+
                                          | watch |
                              +---------> | foo   |
                              |           +-------+
                       +------+-----+
                       |   stream   |
      +--------------> |            |
      |                +------+-----+     +-------+
      |                       |           | watch |
      |                       +---------> | bar   |
+-----+------+                            +-------+
|            |         +------------+
|   conn     +-------> |   stream   |
|            |         |            |
+-----+------+         +------------+
      |
      |
      |
      |                +------------+
      +--------------> |   stream   |
                       |            |
                       +------------+

The theoretical memory consumption of watch can be approximated with the formula: memory = c1 * number_of_conn + c2 * avg_number_of_stream_per_conn + c3 * avg_number_of_watch_stream

Testing Environment

etcd version - git head https://github.com/coreos/etcd/commit/185097ffaa627b909007e772c175e8fefac17af3

GCE n1-standard-2 machine type - 7.5 GB memory - 2x CPUs

Overall memory usage

The overall memory usage captures how much RSS etcd consumes with the client watchers. While the result may vary by as much as 10%, it is still meaningful, since the goal is to learn about the rough memory usage and the pattern of allocations.

With the benchmark result, we can calculate roughly that c1 = 17kb, c2 = 18kb and c3 = 350bytes. So each additional client connection consumes 17kb of memory and each additional stream consumes 18kb of memory, and each additional watching only cause 350bytes. A single etcd server can maintain millions of watchings with a few GB of memory in normal case.

clientsstreams per clientwatchings per streamtotal watchingmemory usage
1k111k50MB
2k112k90MB
5k115k200MB
1k10110k217MB
2k10120k417MB
5k10150k980MB
1k50150k1001MB
2k501100k1960MB
5k501250k4700MB
1k5010500k1171MB
2k50101M2371MB
5k50102.5M5710MB
1k501005M2380MB
2k5010010M4672MB
5k5010025MOOM

© 2019 The etcd authors

Watch Memory Usage Benchmark