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From Reid Pinchback <rpinchb...@tripadvisor.com>
Subject Re: "Maximum memory usage reached (512.000MiB), cannot allocate chunk of 1.000MiB"
Date Mon, 02 Dec 2019 16:07:14 GMT
Rahul, if my memory of this is correct, that particular logging message is noisy, the cache
is pretty much always used to its limit (and why not, it’s a cache, no point in using less
than you have).

No matter what value you set, you’ll just change the “reached (….)” part of it.  I
think what would help you more is to work with the team(s) that have apps depending upon C*
and decide what your performance SLA is with them.  If you are meeting your SLA, you don’t
care about noisy messages.  If you aren’t meeting your SLA, then the noisy messages become
sources of ideas to look at.

One thing you’ll find out pretty quickly.  There are a lot of knobs you can turn with C*,
too many to allow for easy answers on what you should do.  Figure out what your throughput
and latency SLAs are, and you’ll know when to stop tuning.  Otherwise you’ll discover
that it’s a rabbit hole you can dive into and not come out of for weeks.


From: Hossein Ghiyasi Mehr <ghiyasimehr@gmail.com>
Reply-To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Date: Monday, December 2, 2019 at 10:35 AM
To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Subject: Re: "Maximum memory usage reached (512.000MiB), cannot allocate chunk of 1.000MiB"

Message from External Sender
It may be helpful: https://thelastpickle.com/blog/2018/08/08/compression_performance.html<https://urldefense.proofpoint.com/v2/url?u=https-3A__thelastpickle.com_blog_2018_08_08_compression-5Fperformance.html&d=DwMFaQ&c=9Hv6XPedRSA-5PSECC38X80c1h60_XWA4z1k_R1pROA&r=OIgB3poYhzp3_A7WgD7iBCnsJaYmspOa2okNpf6uqWc&m=BlMYluADfxjSCocEBuEfptXuOJCAamgGaQreoJcMRJQ&s=rPo3nouxhBU2Yf2HRb2Udl87roS0KkGuPr-l2ferKXA&e=>
It's complex. Simple explanation, cassandra keeps sstables in memory based on chunk size and
sstable parts. It manage loading new sstables to memory based on requests on different sstables
correctly . You should be worry about it (sstables loaded in memory)

VafaTech.com - A Total Solution for Data Gathering & Analysis


On Mon, Dec 2, 2019 at 6:18 PM Rahul Reddy <rahulreddy1234@gmail.com<mailto:rahulreddy1234@gmail.com>>
wrote:
Thanks Hossein,

How does the chunks are moved out of memory (LRU?) if it want to make room for new requests
to get chunks?if it has mechanism to clear chunks from cache what causes to cannot allocate
chunk? Can you point me to any documention?

On Sun, Dec 1, 2019, 12:03 PM Hossein Ghiyasi Mehr <ghiyasimehr@gmail.com<mailto:ghiyasimehr@gmail.com>>
wrote:
Chunks are part of sstables. When there is enough space in memory to cache them, read performance
will increase if application requests it again.

Your real answer is application dependent. For example write heavy applications are different
than read heavy or read-write heavy. Real time applications are different than time series
data environments and ... .



On Sun, Dec 1, 2019 at 7:09 PM Rahul Reddy <rahulreddy1234@gmail.com<mailto:rahulreddy1234@gmail.com>>
wrote:
Hello,

We are seeing memory usage reached 512 mb and cannot allocate 1MB.  I see this because file_cache_size_mb
by default set to 512MB.

Datastax document recommends to increase the file_cache_size.

We have 32G over all memory allocated 16G to Cassandra. What is the recommended value in my
case. And also when does this memory gets filled up frequent does nodeflush helps in avoiding
this info messages?
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