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From Elliott Sims <elli...@backblaze.com>
Subject Re: Cassandra Recommended System Settings
Date Mon, 21 Oct 2019 21:11:12 GMT
The TCP settings are basically "how much RAM to use to buffer data for TCP
sessions, per session", which translates roughly to maximum TCP window
size.  You can actually calculate approximately what you need by just
multiplying bandwidth and latency (10,000,000,000bps * .0001s * 1GB/8Gb =
125KB buffer needed to fill the pipe).  In practice, I'd double or triple
the max setting vs the calculated value.  The suggested value from Datastax
is 16MB, which doesn't seem like a lot, but if you have 1,0000 connections
that could lead to up to 16GB of RAM being dedicated to TCP buffers.

As an example, my traffic in and out of Cassandra is within a local 10Gb
network.  I use "4096        87380   6291456", but that's not particularly
highly-tuned for Cassandra specifically (that is, it's a value also used by
hosts that talk to the outside internet with much higher latency).

On Mon, Oct 21, 2019 at 1:53 PM Sergio <lapostadisergio@gmail.com> wrote:

> Thanks Elliott!
>
> How do you know if there is too much RAM used for those settings?
>
> Which metrics do you keep track of?
>
> What would you recommend instead?
>
> Best,
>
> Sergio
>
> On Mon, Oct 21, 2019, 1:41 PM Elliott Sims <elliott@backblaze.com> wrote:
>
>> Based on my experiences, if you have a new enough kernel I'd strongly
>> suggest switching the TCP scheduler algorithm to BBR.  I've found the rest
>> tend to be extremely sensitive to even small amounts of packet loss among
>> cluster members where BBR holds up well.
>>
>> High ulimits for basically everything are probably a good idea, although
>> "unlimited" may not be purely optimal for all cases.
>> The TCP keepalive settings are probably only necessary for traffic
>> buggy/misconfigured firewalls, but shouldn't really do any harm on a modern
>> fast network.
>>
>> The TCP memory settings are pretty aggressive and probably result in
>> unnecessary RAM usage.
>> The net.core.rmem_default/net.core.wmem_default settings are overridden
>> by the TCP-specific settings as far as I know, so they're not really
>> relevant/helpful for Cassandra
>> The net.ipv4.tcp_rmem/net.ipv4.tcp_wmem max settings are pretty
>> aggressive.  That works out to something like 1Gbps with 130ms latency per
>> TCP connection, but on a local LAN with latencies <1ms it's enough buffer
>> for over 100Gbps per TCP session.  A much smaller value will probably make
>> more sense for most setups.
>>
>>
>> On Mon, Oct 21, 2019 at 10:21 AM Sergio <lapostadisergio@gmail.com>
>> wrote:
>>
>>>
>>> Hello!
>>>
>>> This is the kernel that I am using
>>> Linux  4.16.13-1.el7.elrepo.x86_64 #1 SMP Wed May 30 14:31:51 EDT 2018
>>> x86_64 x86_64 x86_64 GNU/Linux
>>>
>>> Best,
>>>
>>> Sergio
>>>
>>> Il giorno lun 21 ott 2019 alle ore 07:30 Reid Pinchback <
>>> rpinchback@tripadvisor.com> ha scritto:
>>>
>>>> I don't know which distro and version you are using, but watch out for
>>>> surprises in what vm.swappiness=0 means.  In older kernels it means "only
>>>> use swap when desperate".  I believe that newer kernels changed to have 1
>>>> mean that, and 0 means to always use the oomkiller.  Neither situation is
>>>> strictly good or bad, what matters is what you intend the system behavior
>>>> to be in comparison with whatever monitoring/alerting you have put in place.
>>>>
>>>> R
>>>>
>>>>
>>>> ´╗┐On 10/18/19, 9:04 PM, "Sergio Bilello" <lapostadisergio@gmail.com>
>>>> wrote:
>>>>
>>>>      Message from External Sender
>>>>
>>>>     Hello everyone!
>>>>
>>>>
>>>>
>>>>     Do you have any setting that you would change or tweak from the
>>>> below list?
>>>>
>>>>
>>>>
>>>>     sudo cat /proc/4379/limits
>>>>
>>>>     Limit                     Soft Limit           Hard Limit
>>>>  Units
>>>>
>>>>     Max cpu time              unlimited            unlimited
>>>> seconds
>>>>
>>>>     Max file size             unlimited            unlimited
>>>> bytes
>>>>
>>>>     Max data size             unlimited            unlimited
>>>> bytes
>>>>
>>>>     Max stack size            unlimited            unlimited
>>>> bytes
>>>>
>>>>     Max core file size        unlimited            unlimited
>>>> bytes
>>>>
>>>>     Max resident set          unlimited            unlimited
>>>> bytes
>>>>
>>>>     Max processes             32768                32768
>>>> processes
>>>>
>>>>     Max open files            1048576              1048576
>>>> files
>>>>
>>>>     Max locked memory         unlimited            unlimited
>>>> bytes
>>>>
>>>>     Max address space         unlimited            unlimited
>>>> bytes
>>>>
>>>>     Max file locks            unlimited            unlimited
>>>> locks
>>>>
>>>>     Max pending signals       unlimited            unlimited
>>>> signals
>>>>
>>>>     Max msgqueue size         unlimited            unlimited
>>>> bytes
>>>>
>>>>     Max nice priority         0                    0
>>>>
>>>>     Max realtime priority     0                    0
>>>>
>>>>     Max realtime timeout      unlimited            unlimited
>>>> us
>>>>
>>>>
>>>>
>>>>     These are the sysctl settings
>>>>
>>>>     default['cassandra']['sysctl'] = {
>>>>
>>>>         'net.ipv4.tcp_keepalive_time' => 60,
>>>>
>>>>         'net.ipv4.tcp_keepalive_probes' => 3,
>>>>
>>>>         'net.ipv4.tcp_keepalive_intvl' => 10,
>>>>
>>>>         'net.core.rmem_max' => 16777216,
>>>>
>>>>         'net.core.wmem_max' => 16777216,
>>>>
>>>>         'net.core.rmem_default' => 16777216,
>>>>
>>>>         'net.core.wmem_default' => 16777216,
>>>>
>>>>         'net.core.optmem_max' => 40960,
>>>>
>>>>         'net.ipv4.tcp_rmem' => '4096 87380 16777216',
>>>>
>>>>         'net.ipv4.tcp_wmem' => '4096 65536 16777216',
>>>>
>>>>         'net.ipv4.ip_local_port_range' => '10000 65535',
>>>>
>>>>         'net.ipv4.tcp_window_scaling' => 1,
>>>>
>>>>            'net.core.netdev_max_backlog' => 2500,
>>>>
>>>>            'net.core.somaxconn' => 65000,
>>>>
>>>>         'vm.max_map_count' => 1048575,
>>>>
>>>>         'vm.swappiness' => 0
>>>>
>>>>     }
>>>>
>>>>
>>>>
>>>>     Am I missing something else?
>>>>
>>>>
>>>>
>>>>     Do you have any experience to configure CENTOS 7
>>>>
>>>>     for
>>>>
>>>>     JAVA HUGE PAGES
>>>>
>>>>
>>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__docs.datastax.com_en_dse_5.1_dse-2Dadmin_datastax-5Fenterprise_config_configRecommendedSettings.html-23CheckJavaHugepagessettings&d=DwIBaQ&c=9Hv6XPedRSA-5PSECC38X80c1h60_XWA4z1k_R1pROA&r=OIgB3poYhzp3_A7WgD7iBCnsJaYmspOa2okNpf6uqWc&m=zke-WpkD1c6Qt1cz8mJG0ZQ37h8kezqknMSnerQhXuU&s=b6lGdbtv1SN9opBsIOFRT6IX6BroMW-8Tudk9qEh3bI&e=
>>>>
>>>>
>>>>
>>>>     OPTIMIZE SSD
>>>>
>>>>
>>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__docs.datastax.com_en_dse_5.1_dse-2Dadmin_datastax-5Fenterprise_config_configRecommendedSettings.html-23OptimizeSSDs&d=DwIBaQ&c=9Hv6XPedRSA-5PSECC38X80c1h60_XWA4z1k_R1pROA&r=OIgB3poYhzp3_A7WgD7iBCnsJaYmspOa2okNpf6uqWc&m=zke-WpkD1c6Qt1cz8mJG0ZQ37h8kezqknMSnerQhXuU&s=c0S3S3V_0YHVMx2I-pyOh24MiQs1D-L73JytaSw648M&e=
>>>>
>>>>
>>>>
>>>>
>>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__docs.datastax.com_en_dse_5.1_dse-2Dadmin_datastax-5Fenterprise_config_configRecommendedSettings.html&d=DwIBaQ&c=9Hv6XPedRSA-5PSECC38X80c1h60_XWA4z1k_R1pROA&r=OIgB3poYhzp3_A7WgD7iBCnsJaYmspOa2okNpf6uqWc&m=zke-WpkD1c6Qt1cz8mJG0ZQ37h8kezqknMSnerQhXuU&s=PZFG6SXF6dL5LRJ-aUoidHnnLGpKPbpxdKstM8M9JMk&e=
>>>>
>>>>
>>>>
>>>>     We are using AWS i3.xlarge instances
>>>>
>>>>
>>>>
>>>>     Thanks,
>>>>
>>>>
>>>>
>>>>     Sergio
>>>>
>>>>
>>>>
>>>>
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>>>>
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>>>>
>>>>
>>>>
>>>>
>>>>

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