How much free memory does Redis need to run? - memory

I'm pretty sure at this stage that Redis needs a certain amount of free memory on the OS in order to run. In the past few weeks, I've seen Redis (Linux) run out of memory with a couple of gigabytes of RAM still free, and on Windows, it refuses to start when you are using a lot of memory on the system but still have a bunch left free, as in the screenshot below.
The error on Windows gives a hint as to why this is happening (although I'm not assuming it's the same on Linux). However, my question is more generic. How much free memory does Redis need in order to operate?

Redis requires RAM between x2 to x3 the size of your data. The maxheap flag is Windows-specific.

According to Redis FAQ, without a specific Linux configuration, it might need 2x the memory of your dataset. From the document:
Short answer: echo 1 > /proc/sys/vm/overcommit_memory :)
With this configuration, the forked process (responsible for saving the dataset to disk) will be able to share memory pages more easily with the original process, so it won't need that much memory.
You can read more about this here: https://redis.io/topics/faq#background-saving-fails-with-a-fork-error-under-linux-even-if-i-have-a-lot-of-free-ram

Related

Why is using host memory recommended by docker

I am investigating a topic, which I will call “Docker swarm and memory management”.
It states in this article here that docker does not recommend using swap memory, but I can’t find (googling) a place where disadvantages of using swap memory in docker context is explained.
Can a kind soul enlighten me? :-)
It is normal to disable SWAP memory in ALL applications or services that are used in production.
SWAP memory is based on using the hard disk as a substitute when the RAM is full. This may seem beneficial but the RAM has a speed from 2.1 GB/s the oldest to 25.6 GB/s the newest. Contrary to the speed of a hard drive with HDDs on average at 135MB/s, newer M.2 SSDs at 1.2GB/s.
As we can see we would be greatly slowing down the service if we were using SWAP.

AWS server became slow after traffic increase

I have a single page Angular app that makes request to a Rails API service. Both are running on a t2xlarge Ubuntu instance. I am using a Postgres database.
We had increase in traffic, and my Rails API became slow. Sometimes, I get an error saying Passenger queue full for rails application.
Auto scaling on the server is working; three more instances are created. But I cannot trace this issue. I need root access to upgrade, which I do not have. Please help me with this.
As you mentioned that you are using T2.2xlarge instance type. Firstly I want to tell you should not use T2 instance type for production environment. Cause of T2 instance uses CPU Credit. Lets take a look on this
What happens if I use all of my credits?
If your instance uses all of its CPU credit balance, performance
remains at the baseline performance level. If your instance is running
low on credits, your instance’s CPU credit consumption (and therefore
CPU performance) is gradually lowered to the base performance level
over a 15-minute interval, so you will not experience a sharp
performance drop-off when your CPU credits are depleted. If your
instance consistently uses all of its CPU credit balance, we recommend
a larger T2 size or a fixed performance instance type such as M3 or
C3.
Im not sure you won't face to the out of CPU Credit problem because you are using Xlarge type but I think you should use other fixed performance instance types. So instance's performace maybe one part of your problem. You should use cloudwatch to monitor on 2 metrics: CPUCreditUsage and CPUCreditBalance to make sure the problem.
Secondly, how about your ASG? After scale-out, did your service become stable? If so, I think you do not care about this problem any more because ASG did what it's reponsibility.
Please check the following
If you are opening a connection to Database, make sure you close it.
If you are using jquery, bootstrap, datatables, or other css libraries, use the CDN links like
<link rel="stylesheet" ref="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.12.4/css/bootstrap-select.min.css">
it will reduce a great amount of load on your server. do not copy the jquery or other external libraries on your own server when you can directly fetch it from other servers.
There are a number of factors that can cause an EC2 instance (or any system) to appear to run slowly.
CPU Usage. The higher the CPU usage the longer to process new threads and processes.
Free Memory. Your system needs free memory to process threads, create new processes, etc. How much free memory do you have?
Free Disk Space. Operating systems tend to thrash when the file systems on system drives run low on free disk space. How much free disk space do you have?
Network Bandwidth. What is the average bytes in / out for your
instance?
Database. Monitor connections, free memory, disk bandwidth, etc.
Amazon has CloudWatch which can provide you with monitoring for everything except for free disk space (you can add an agent to your instance for this metric). This will also help you quickly see what is happening with your instances.
Monitor your EC2 instances and your database.
You mention T2 instances. These are burstable CPUs which means that if you have consistenly higher CPU usage, then you will want to switch to fixed performance EC2 instances. CloudWatch should help you figure out what you need (CPU or Memory or Disk or Network performance).
This is totally independent of AWS Server. Looks like your software needs more juice (RAM, StorageIO, Network) and it is not sufficient with one machine. You need to evaluate the metric using cloudwatch and adjust software needs based on what is required for the software.
It could be memory leaks or processing leaks that may lead to this as well. You need to create clusters or server farm to handle the load.
Hope it helps.

What are the minimum requirements of neo4j?

I'd like to use a neo4j database in a docker container with Odroid XU4. The database is not big, approximately 20.000 nodes will be in it. The Odroid has only 2G memory, and I'd like to have a samba server, some nodejs applications and at least one PgSQL database too, so the system is short on memory. I read in the neo4j manual that 2G memory is the minimum, but I read by docker examples that it is used with 512M, so I am a little confused about this. What is the minimum memory I can use the neo4j docker image with?
I have similar troubles with the disk space. The system is on a 32GB SD card. I'd like to save database data there and backup on an external hard drive, so I could spend max 16GB for the neo4j. The data certainly does not require that kind of space, I am not sure why neo4j needs it (according to the manual again).
First you can use http://neo4j.com/hardware-sizing-calculator/ to get rough estimate for memory and disk usage.
Second option is to do some math. You can use information on page 12 in http://graphaware.com/assets/bachman-msc-thesis.pdf
You should keep in mind it's good to have all data in the memory for the performance reasons.
From my point of view you shouldn't have problem with the memory, but you can't expect great performance.
It's better to try it by yourself before you ask here ;)

Digital Ocean server memory usage above 50%

I am deploying a Flask-based website on the server of Digital Ocean. And the website deployed is mainly static pages, config files and jsons.
This morning I found the memory usage has exceeded 51%. Here is the snapshot.
My memory is 512MB. Would someone please instruct me how to lower the memory usage? Thanks so much!
Update: I've use the "top" command in shell as suggested. Here is the snapshot, does it mean that it is the server itself eaten up those memories?
The memory issue is not related to my application.
I just received the answer from Digital Ocean. Here it is:
Hi there!
Thank you for contacting us! We can help with any memory issues you're having!
Since the Droplet is set up with only 512MB of RAM, once the system and any installed services start, it doesn't take much to push it past 50%. As a result, I don't think what you're seeing is necessarily abnormal under the circumstances. This leaves a few options: the Droplet can be resized and made larger to provide more memory (see https://www.digitalocean.com/community/tutorials/how-to-resize-your-droplets-on-digitalocean), you can add swap space to use part of the Droplet's file system as RAM (see https://www.digitalocean.com/community/tutorials/how-to-add-swap-on-ubuntu-14-04), or you can review the applications and services running on the Droplet and attempt to optimize them to reduce memory use.
We hope this is helpful! Please let us know if there is anything else we can do!
Regards,
I am assuming your are running a Linux server. If so, you can use the top command. It shows you all of the running processes and the system resources they are using. You would then be able to optimize from there.
I found out the cause! Linux borrows unused memory for disk caching. This makes it look like you are low on memory, but you are not! Everything is fine! If your application, or any other process needs more memory, Linux will automatically clear the cache and give memory for your application. Linux does this to speed up the system for you.
If, however, you find yourself needing to clear some RAM quickly to workaround another issue, like a VM misbehaving, you can force Linux to nondestructively drop caches using:
echo 3 | sudo tee /proc/sys/vm/drop_caches

Is it worth to install win ce cf 3.5 application on storage card?

Since our application grows, we need more space on our Windows CE devices.
If I install CF app in RAM on win ce device this app vanished after cold restart.
I have used the simplest choice install on flash card. As I mentioned running applications from the sd card is slow and there are some heavy issues with demand-paging if you run the apps from persistent paths. Isn't it? Is it worth to install it there? Will we get performance problems?
Should I use another solution - install after cold restart/new start on RAM from flash disk (if it possible)? Where can/should I store settings/log files? On flash/sd card?
There's no "one size fits all" answer for this.
If you move the app from memory to storage you'll gain RAM. Maybe that boost in RAM will give the EE more heap space and thereby prevent GC thrashing. That would give you better perceived performance. But maybe it won't and it will just increase demand-paging for your app and hurt performance. Maybe you'll get a little of both and it's a wash.
How would you handle persistence to RAM? That depends on what your device supports for auto-running apps.
Where should you store settings and logs? Again, that depends on the device, the storage, the size, the frequency of access and loads of other things.
Basically the answer for all of these is only going to be found by you testing your actual app on your actual hardware. Try the difference scenarios and collect metrics to see which performs better. That's the only "correct" answer.

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