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. >>>>> >>>>>
