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From Jon Haddad <...@jonhaddad.com>
Subject Re: Cassandra 2.1.18 - Question on stream/bootstrap throughput
Date Tue, 22 Oct 2019 16:32:08 GMT
CPU waiting on memory will look like CPU overhead.   There's a good post on
the topic by Brendan Gregg:
http://www.brendangregg.com/blog/2017-05-09/cpu-utilization-is-wrong.html

Regarding GC, I agree with Reid.  You're probably not going to saturate
your network card no matter what your settings, Cassandra has way too much
overhead to do that.  It's one of the reasons why the whole zero-copy
streaming feature was added to Cassandra 4.0:
http://cassandra.apache.org/blog/2018/08/07/faster_streaming_in_cassandra.html

Reid is also correct in pointing out the method by which you're monitoring
your metrics might be problematic.  With prometheus, the same data can show
significantly different graphs when using rate vs irate, and only
collecting once a minute would hide a lot of useful data.

If you keep digging and find you're not using all your CPU during GC
pauses, you can try using more GC threads by setting -XX:ParallelGCThreads
to match the number of cores you have, since by default it won't use them
all.  You've got 40 cores in the m4.10xlarge, try
setting -XX:ParallelGCThreads to 40.

Jon



On Tue, Oct 22, 2019 at 11:38 AM Reid Pinchback <rpinchback@tripadvisor.com>
wrote:

> Thomas, what is your frequency of metric collection?  If it is
> minute-level granularity, that can give a very false impression.  I’ve seen
> CPU and disk throttles that don’t even begin to show visibility until
> second-level granularity around the time of the constraining event.  Even
> clearer is 100ms.
>
>
>
> Also, are you monitoring your GC activity at all?  GC bound up in a lot of
> memory copies is not going to manifest that much CPU, it’s memory bus
> bandwidth you are fighting against then.  It is easy to have a box that
> looks unused but in reality its struggling.  Given that you’ve opened up
> the floodgates on compaction, that would seem quite plausible to be what
> you are experiencing.
>
>
>
> *From: *"Steinmaurer, Thomas" <thomas.steinmaurer@dynatrace.com>
> *Reply-To: *"user@cassandra.apache.org" <user@cassandra.apache.org>
> *Date: *Tuesday, October 22, 2019 at 11:22 AM
> *To: *"user@cassandra.apache.org" <user@cassandra.apache.org>
> *Subject: *RE: Cassandra 2.1.18 - Question on stream/bootstrap throughput
>
>
>
> *Message from External Sender*
>
> Hi Alex,
>
>
>
> Increased streaming throughput has been set on the existing nodes only,
> cause it is meant to limit outgoing traffic only, right? At least when
> judging from the name, reading the documentation etc.
>
>
>
> Increased compaction throughput on all nodes, although my understanding is
> that it would be necessary only on the joining node to catchup with
> compacting received SSTables.
>
>
>
> We really see no resource (CPU, NW and disk) being somehow maxed out on
> any node, which would explain the limit in the area of the new node
> receiving data at ~ 180-200 Mbit/s.
>
>
>
> Thanks again,
>
> Thomas
>
>
>
> *From:* Oleksandr Shulgin <oleksandr.shulgin@zalando.de>
> *Sent:* Dienstag, 22. Oktober 2019 16:35
> *To:* User <user@cassandra.apache.org>
> *Subject:* Re: Cassandra 2.1.18 - Question on stream/bootstrap throughput
>
>
>
> On Tue, Oct 22, 2019 at 12:47 PM Steinmaurer, Thomas <
> thomas.steinmaurer@dynatrace.com> wrote:
>
>
>
> using 2.1.8, 3 nodes (m4.10xlarge, ESB SSD-based), vnodes=256, RF=3, we
> are trying to add a 4th node.
>
>
>
> The two options to my knowledge, mainly affecting throughput, namely
> stream output and compaction throttling has been set to very high values
> (e.g. stream output = 800 Mbit/s resp. compaction throughput = 500 Mbyte/s)
> or even set to 0 (unthrottled) in cassandra.yaml + process restart. In both
> scenarios (throttling with high values vs. unthrottled), the 4th node is
> streaming from one node capped ~ 180-200Mbit/s, according to our SFM.
>
>
>
> The nodes have plenty of resources available (10Gbit, disk io/iops), also
> confirmed by e.g. iperf in regard to NW throughput and write to / read from
> disk in the area of 200 MByte/s.
>
>
>
> Are there any other known throughput / bootstrap limitations, which
> basically outrule above settings?
>
>
>
> Hi Thomas,
>
>
>
> Assuming you have 3 Availability Zones and you are adding the new node to
> one of the zones where you already have a node running, it is expected that
> it only streams from that node (its local rack).
>
>
>
> Have you increased the streaming throughput on the node it streams from or
> only on the new node?  The limit applies to the source node as well.  You
> can change it online w/o the need to restart using nodetool command.
>
>
>
> Have you checked if the new node is not CPU-bound?  It's unlikely though
> due to big instance type and only one node to stream from, more relevant
> for scenarios when streaming from a lot of nodes.
>
>
>
> Cheers,
>
> --
>
> Alex
>
>
>
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