I tried running two client processes in parallel and the numbers were
unchanged. The max throughput is still a single client doing 10 in-flight
BatchStatement containing 100 inserts.

On Tue, Apr 23, 2024 at 10:24 PM Bowen Song via user <
user@cassandra.apache.org> wrote:

> You might have run into the bottleneck of the driver's IO thread. Try
> increase the driver's connections-per-server limit to 2 or 3 if you've only
> got 1 server in the cluster. Or alternatively, run two client processes in
> parallel.
> On 24/04/2024 07:19, Nathan Marz wrote:
> Tried it again with one more client thread, and that had no effect on
> performance. This is unsurprising as there's only 2 CPU on this node and
> they were already at 100%. These were good ideas, but I'm still unable to
> even match the performance of batch commit mode with group commit mode.
> On Tue, Apr 23, 2024 at 12:46 PM Bowen Song via user <
> user@cassandra.apache.org> wrote:
>> To achieve 10k loop iterations per second, each iteration must take 0.1
>> milliseconds or less. Considering that each iteration needs to lock and
>> unlock the semaphore (two syscalls) and make network requests (more
>> syscalls), that's a lots of context switches. It may a bit too much to ask
>> for a single thread. I would suggest try multi-threading or
>> multi-processing, and see if the combined insert rate is higher.
>> I should also note that executeAsync() also has implicit limits on the
>> number of in-flight requests, which default to 1024 requests per connection
>> and 1 connection per server. See
>> https://docs.datastax.com/en/developer/java-driver/4.17/manual/core/pooling/
>> On 23/04/2024 23:18, Nathan Marz wrote:
>> It's using the async API, so why would it need multiple threads? Using
>> the exact same approach I'm able to get 38k / second with periodic
>> commitlog_sync. For what it's worth, I do see 100% CPU utilization in every
>> single one of these tests.
>> On Tue, Apr 23, 2024 at 11:01 AM Bowen Song via user <
>> user@cassandra.apache.org> wrote:
>>> Have you checked the thread CPU utilisation of the client side? You
>>> likely will need more than one thread to do insertion in a loop to achieve
>>> tens of thousands of inserts per second.
>>> On 23/04/2024 21:55, Nathan Marz wrote:
>>> Thanks for the explanation.
>>> I tried again with commitlog_sync_group_window at 2ms, concurrent_writes
>>> at 512, and doing 1000 individual inserts at a time with the same loop +
>>> semaphore approach. This only nets 9k / second.
>>> I got much higher throughput for the other modes with BatchStatement of
>>> 100 inserts rather than 100x more individual inserts.
>>> On Tue, Apr 23, 2024 at 10:45 AM Bowen Song via user <
>>> user@cassandra.apache.org> wrote:
>>>> 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 documentation
>>>>>> <https://cassandra.apache.org/doc/stable/cassandra/architecture/storage_engine.html>
>>>>>> 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
>>>>>> 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) {
>>>>>> sem.acquire();
>>>>>> session.executeAsync(insert.bind(genUUIDStr(), 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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