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. >>>> >>>>
