December 31, 2021 in Technology, Tips and Tricks

how can identify client computer uniquely for website users

Cookies

Set a cookie on the client machine to identify the user.

• Pros: Definitely unique per session
• Cons: Easy to remove or prevent (even for people with limited technical knowledge), only apply to one browser, easy for a script to bypass

IP address

Use the user’s IP address to identify the user.

• Pros: Changes infrequently, works across browsers, requires technical knowledge to spoof or use a proxy server
• Cons: Not necessarily unique: one IP address can easily apply to entire homes/buildings/offices behind a network, and for some ISPs end user IP addresses might be reassigned

Browser fingerprinting

Use all browser data passed to the server (not just user agent, but also OS, screen resolution, and a number of other things) to generate a unique browser fingerprint.

• Pros: High probability of being unique, opaque to the end user (changing IP addresses or clearing cookies won’t bypass it)
• Cons: Probably too unique — any change in browser condition (or browser) will change the fingerprint (though you can choose only a subset of supplied browser information to balance fingerprint volatility and uniqueness), can be bypassed by scripts

You can, of course, also use multiple methods. Which method you use really depends on your specific use case. For many cases, just a simple cookie will do. If you want some defense against scripts or other methods of bypassing, add an IP address rate limit. You can also attempt to gather as much information as possible and use some sort of heuristic algorithm to try to find people that are bypassing the system for uniqueness that you have in place.

December 7, 2021 in Technology, Tips and Tricks

buffer cache in linux is increase and get ram usage

Don’t Panic!
Your ram is fine!

What’s going on?

Linux is borrowing unused memory for disk caching. This makes it look like you are low on memory, but you are not! Everything is fine!

Why is it doing this?

Disk caching makes the system much faster and more responsive! There are no downsides, except for confusing newbies. It does not take memory away from applications in any way, ever!

What if I want to run more applications?

If your applications want more memory, they just take back a chunk that the disk cache borrowed. Disk cache can always be given back to applications immediately! You are not low on ram!

Do I need more swap?

No, disk caching only borrows the ram that applications don’t currently want. It will not use swap. If applications want more memory, they just take it back from the disk cache. They will not start swapping.

How do I stop Linux from doing this?

You can’t disable disk caching. The only reason anyone ever wants to disable disk caching is because they think it takes memory away from their applications, which it doesn’t! Disk cache makes applications load faster and run smoother, but it NEVER EVER takes memory away from them! Therefore, there’s absolutely no reason to disable it!

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.

Why does top and free say all my ram is used if it isn’t?

This is just a difference in terminology. Both you and Linux agree that memory taken by applications is “used”, while memory that isn’t used for anything is “free”.

But how do you count memory that is currently used for something, but can still be made available to applications?

You might count that memory as “free” and/or “available”. Linux instead counts it as “used”, but also “available”:

This “something” is (roughly) what top and free calls “buffers” and “cached”. Since your and Linux’s terminology differs, you might think you are low on ram when you’re not.

How do I see how much free ram I really have?

To see how much ram your applications could use without swapping, run free -m and look at the “available” column:

  $ free -m
                total        used        free      shared  buff/cache   available
  Mem:           1504        1491          13           0         855      792
  Swap:          2047           6        2041

(On installations from before 2016, look at “free” column in the “-/+ buffers/cache” row instead.)

This is your answer in MiB. If you just naively look at “used” and “free”, you’ll think your ram is 99% full when it’s really just 47%!

For a more detailed and technical description of what Linux counts as “available”, see the commit that added the field.

When should I start to worry?

A healthy Linux system with more than enough memory will, after running for a while, show the following expected and harmless behavior:

• free memory is close to 0
• used memory is close to total
• available memory (or “free + buffers/cache”) has enough room (let’s say, 20%+ of total)
• swap used does not change

Warning signs of a genuine low memory situation that you may want to look into:

• available memory (or “free + buffers/cache”) is close to zero
• swap used increases or fluctuates
• dmesg | grep oom-killer shows the OutOfMemory-killer at work

How can I verify these things?

See this page for more details and how you can experiment with disk cache to show the effects described here. Few things make you appreciate disk caching more than measuring an order-of-magnitude speedup on your own hardware!