Okay, that proves I was wrong on the client side bottleneck.
On 24/04/2024 17:55, Nathan Marz wrote:
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.
