I suspect you are abusing batch statements. Batch statements should only be used where atomicity or isolation is needed. Using batch statements won't make inserting multiple partitions faster. In fact, it often will make that slower.

Also, the liner relationship between commitlog_sync_group_window and write throughput is expected. That's because the max number of uncompleted writes is limited by the write concurrency, and a write is not considered "complete" before it is synced to disk when commitlog sync is in group or batch mode. That means within each interval, only limited number of writes can be done. The ways to increase that including: add more nodes, sync commitlog at shorter intervals and allow more concurrent writes.

On 23/04/2024 20:43, Nathan Marz wrote:
Thanks. I raised concurrent_writes to 128 and set commitlog_sync_group_window to 20ms. This causes a single execute of a BatchStatement containing 100 inserts to succeed. However, the throughput I'm seeing is atrocious.

With these settings, I'm executing 10 BatchStatement concurrently at a time using the semaphore + loop approach I showed in my first message. So as requests complete, more are sent out such that there are 10 in-flight at a time. Each BatchStatement has 100 individual inserts. I'm seeing only 730 inserts / second. Again, with periodic mode I see 38k / second and with batch I see 14k / second. My expectation was that group commit mode throughput would be somewhere between those two.

If I set commitlog_sync_group_window to 100ms, the throughput drops to 14 / second.

If I set commitlog_sync_group_window to 10ms, the throughput increases to 1587 / second.

If I set commitlog_sync_group_window to 5ms, the throughput increases to 3200 / second.

If I set commitlog_sync_group_window to 1ms, the throughput increases to 13k / second, which is slightly less than batch commit mode.

Is group commit mode supposed to have better performance than batch mode?

On Tue, Apr 23, 2024 at 8:46 AM Bowen Song via user <user@cassandra.apache.org> wrote:

    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
    <user@cassandra.apache.org> wrote:

        Why would you want to set commitlog_sync_batch_window to 1
        second long when commitlog_sync is set to batch mode? The
        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

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

        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) {

            genUUIDStr(), genUUIDStr())
                        .whenComplete((t, u) -> sem.release())


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

        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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