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.
