Tuesday, May 13, 2008

Net Settings mySQL & Memcache

Ever see this
TCP: drop open request from 10.209.23.142/43407


Well lets start with a more specific example:
Memcache is tightly coupled in your code: Every request caches the response from the database so a lot of quick calls to memcache is made. Then you start adding full HTML to memcache instead of just caching the raw data; so now your load pattern is bigger blobs of data still at a high request rate.

Now suddenly the memcache port hangs-you verify this by ssh to the box and then telneting the the memcache box port 11211 and see that ssh works (port 22) yet 11211 does not. As a result all your front ends fall over because they are hanging on the memcache port.

THIS IS NOT A MEMCACHE PROBLEM. Its a kernel problem. Default installs of Linux set the TCP window buffer size to a desktop setting and not a server setting.

So I run this script.


#!/bin/bash

sysctl -w net/core/rmem_max=8738000
sysctl -w net/core/wmem_max=6553600

sysctl -w net/ipv4/tcp_rmem="8192 873800 8738000"
sysctl -w net/ipv4/tcp_wmem="4096 655360 6553600"
sysctl -w vm/min_free_kbytes=65536



I found this out by going here

This is the first kernel setting that I have seen make a real big difference.


So my list of changes so far to the kernel default settings are (getting lazy in detail)

vm.swappiness=0
run the deadline scheduler

On the filesystem side

mount the datadir noatime
use O_DIRECT
if you have cache on a hardware raid card set the cache for writes (make sure you have a BBC)
use Raid-10 or if you have the money + can take a hit on I/O RAID-6
stripe size 128-256K

I have some other tweaks that I'm forgetting but when I find them I'll post them.

8 comments:

Steffen Weber said...

An alternative way to solve the memcached problem should be to use UNIX domain sockets instead of TCP/IP.

For example in PHP you can use:

$memcache->connect('unix:///var/run/memcached/memcached.sock', 0)

George said...

For an added bonus, put your datadir on an unjournalled FS. Since you already effectively have a journal in the innodb TX log, you dont /really/ need another FS one.

If you do have an FS journal, you can rebuild the fs to have a much bigger journal and flush it less often.

Dathan said...

I always wondered about that. I've been very cautious to run EXT2 (no FS journal), yet doing this does make sense.

I'm glad to see others running their systems like this.

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Drew said...

The one catch with your setup I believe is that there's a known problem with swapiness=0. If you have a lot of memory (e.g. 16 GB), the memory will start swapping even when it's only partially used. swapiness=10 is a better option

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