The default commitlog_sync_group_window is very long for SSDs. Try reduce it if you are using SSD-backed storage for the commit log. 10-15 ms is a good starting point. You may also want to increase the value of concurrent_writes, consider at least double or quadruple it from the default. You'll need even higher write concurrency for longer commitlog_sync_group_window.

On 23/04/2024 19:26, Nathan Marz wrote:
"batch" mode works fine. I'm having trouble with "group" mode. The only config for that is "commitlog_sync_group_window", and I have that set to the default 1000ms.

On Tue, Apr 23, 2024 at 8:15 AM Bowen Song via user <> wrote:

    Why would you want to set commitlog_sync_batch_window to 1 second
    long when commitlog_sync is set to batch mode? The documentation
    on this says:

        /This window should be kept short because the writer threads
        will be unable to do extra work while waiting. You may need to
        increase concurrent_writes for the same reason/

    If you want to use batch mode, at least ensure
    commitlog_sync_batch_window is reasonably short. The default is 2

    On 23/04/2024 18:32, Nathan Marz wrote:
    I'm doing some benchmarking of Cassandra on a single m6gd.large
    instance. It works fine with periodic or batch commitlog_sync
    options, but I'm having tons of issues when I change it to
    "group". I have "commitlog_sync_group_window" set to 1000ms.

    My client is doing writes like this (pseudocode):

    Semaphore sem = new Semaphore(numTickets);
    while(true) {

        session.executeAsync(insert.bind(genUUIDStr(), genUUIDStr(),
                    .whenComplete((t, u) -> sem.release())


    If I set numTickets higher than 20, I get tons of timeout errors.

    I've also tried doing single commands with BatchStatement with
    many inserts at a time, and that fails with timeout when the
    batch size gets more than 20.

    Increasing the write request timeout in cassandra.yaml makes it
    time out at slightly higher numbers of concurrent requests.

    With periodic I'm able to get about 38k writes / second, and with
    batch I'm able to get about 14k / second.

    Any tips on what I should be doing to get group commitlog_sync to
    work properly? I didn't expect to have to do anything other than
    change the config.

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