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.
