If you are running your services on a low end virtual hosting every byte of memory you can save is important. The memory is often the limiting factor of how many applications you can run on VPS: CPUs are shared, memory not, on the same physical host.
- Low-end VPS come with 512 MB memory or less
- Front front-end server Apache / Nginx / Varnish takes > 100 MB + min. 20 MB for each child process
- Memecached takes its toll
- MySQL takes 200 – 400 MB
- Each Python / PHP process takes at least 15 MB and you need parallel processes for paraller HTTP requests (FCGI, pre-fork, others… )
- Operating system processes need some memory (SSH, cron, sendmail)
As you can see it gets very crowded in 512 MB.
It’s especially troublesome since the memory is allocated lazily and the memory usage builds up slowly. In some point caches are no longer caches, but swapped to a disk – virtual memory usage grows beyond available RAM. To keep the server response, everything time critical should fit to RAM once and if the processes themselves don’t know how to release memory in this situation you need to tune a memory cap for them.
1. MySQL memory consumption
MySQL can be a greedy bastard what comes to memory consumption. Here on this server MySQL seems to take 417M virtual memory which seems to be little excessive for just running two WordPress instances and one Django / Python application:
1310 mysql 20 0 417M 21100 2776 S 0.0 1.2 0:00.00 /usr/sbin/mysqld --basedir=/usr --datadir=/var/lib/mysql --user=mysql --pid-file=/v
After some tuning I was able to bring it down a bit
3354 mysql 20 0 276m 19m 2848 S 0 1.2 3:41.19 mysqld
A reduction of 130 MB, or 1/4 of the server total memory. Not bad.
Use mtop to monitor running MySQL, its querieries, etc. so you know what’s going on. As you can see this MySQL has very good cache rate meaning that basically it is keeping everything in memory. If the content of the sites is less than 10 MBytes total, 400 MB contains plenty of space to cache the content:
load average: 0.05, 0.08, 0.16 mysqld 5.0.51a-3ubuntu5.8-log up 1 day(s), 19:47 hrs 2 threads: 1 running, 6 cached. Queries/slow: 187.1K/0 Cache Hit: 99.39%
2. What eats memory
I am not an expert on MySQL, so I hope someone with more insight could post comments regarding how to tune MySQL for low memory situations and how it is expected to behave.
Some ideas I run through my head
- MySQL default cache settings are not too tight on Ubuntu/Debian, making it suitable for moderate loads, not low loads. If you don’t have much content, everything is just kept in memory (even if not needed)
- MySQL uses round robin for connections and if there is 100 max connections it will allocate a thread stack for each connection (someone please confirm this – I found contracting infos).
3. Configuring MySQL
Here are listed some methods how to reduce the memory usage. This is what I done on this little box
MySQL is mostly configured in /etc/mysql/my.cnf on Ubuntu / Debian.
- Let’s halve key_buffer from 16M to 8M. It is used by MyISAM table cache.
- Halve query_cache_size. query_cache_size = 8M. Also, decrease query_cache_limit to 512 K.
- Each connection, even if idle, will have 256 KB buffer. Decrease the number of max. connections. (XXX: not sure about this). Drop max connections from 100 -> 30, as we do not have that many concurrent visitors on the site. Also, set less aggressive thread_stack size.
The final adjustments
key_buffer = 8M max_connections = 30 query_cache_size = 8M query_cache_limit = 512K thread_stack = 128K
4. More info
- Fine-tuning MySQL
- Showing what database engine MySQL tables are using
- MySQL server memory usage
- Reducing MySQL memory usage for low end boxes
Send in more tips please! Is 32-bit better than 64-bit for low end VPS, how much this affects MySQL?
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Hi Mikko,
The problem you are facing is not ‘mysql memory usage’ problem, that’s OpenVZ memory management issue.
In your example:
1310 mysql 20 0 417M 21100 2776 S 0.0 1.2 0:00.00 /usr/sbin/mysqld
mysql uses 21M RSS, not 417M. 417M is VIRT memory, it given for free on almost all systems (on real servers, on xen or kvm vps’es) so software usually can allocate as much VIRT memory as it wants. Moreover, OS itself consider VIRT free to allocate and allocates about 10M per thread for thread stack in VIRT memory. But this doesn’t work in OpenVZ because VIRT is limited here, not RSS.
That’s why many-many people consider apache or mysql InnoDB memory hog. These programs are using threads and on cheap OpenVZ VPS’es they are eating huge amount of memory indeed.
There are several possible ways to decrease memory usage on OpenVZ.
1. Use single-thereaded software (e.g. replace apache with nginx, use myisam instead of innodb)
2. Decrease thread stack using software config options (e.g. ThreadStackSize option for apache)
3. Decrease thread stack size globally. This can be done with ‘ulimit -s ‘ command. Try e.g. typing ‘ulimit -s 2048’, then restart mysql and check memory usage. If all is working fine, this command can be added to booting sequence (e.g. to /etc/init.d/rc so it run before starting mysql and apache).
But the best way is to avoid OpenVZ and use XEN/KVM. “512M OpenVZ” is a pure marketing, it is worse than 256M XEN because that’s just different memory. Look, your mysql will use 400M+ on OpenVZ and less than 25M on XEN. Get something like linode and your memory problems will disappear.
There must be some sort of DB-connection pooling (app?), for Django’s ORM? Then you can have a tunable, fixed # of DB connections, released to the pool when a request is completed…
I would think there’s a PHP DB-connection pooling “app”, too.
Do you really want to run a Varnish caching proxy, and “memcached”, in that relatively small VPS? Perhaps there’s a way to use a Nginx plugin, and Django caching, to take over from these? (not sure).
Thank you for your insightful reply Mikhai. However, I am not using OpenVZ and I am using XEN.
If you are using XEN then memory consumption shouldn’t be measured with VIRT column 😉 In your case Mysql consumes 21M before changes and 19M after, not 417M and 276M.
VIRT still goes to swap and means that it is put on the disk in some point, right?
No, VIRT does not goes to swap and in most cases it is never put on disk. Swapped memory accounts in VIRT as well as stacks, mmaped files, etc. Take a look at http://linux-mm.org/ActualMemoryFootprint for more info. Gigabytes of VIRT can be allocated without negative impact on system.
However, tn this particular case we can observe swap space growth, so something is going there and memory usage must be reduced 🙂
Yep, but in order to reduce memory it’s good to know what is exactly consuming memory. And in order to know this take a look at RSS column, not at VIRT column (on XEN).
Increasing VIRT can be a side effect of swapping, but it is definitely not a cause of swapping.
Any tools for getting better data, with historic profile, how much Linux processes hold memory in RES and how much is swapped out?
Some good info plus link to (hopefully usable) tool, unfortunately not packaged.
http://ktown.kde.org/~seli/memory/analysis.html
Use mysqltuner http://mysqltuner.pl/mysqltuner.pl to check real requirements of your database and fine-tune memory-related parameters.
After using Mikko’s suggestions for MySQL settings, there is a noticeable difference. Although I am on shared hosting and not a VPS.
@Mikko thanks for the nginx tip!
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“less than 10 MBytes total,”
???
a site that small doesn’t need mysql ..
if its that small,put the data in files one record per line and use grep!
it would be MUCH faster than ANY database!
but of course that won’t scale so well to a few GB
Then mysql would normally be a better choice .. but nowdays am starting to wonder with the excessive ram use and constant swap making everything go slow..
I suspect that the developers assumed a mysql instance will be on it’s own box …
but I’m out here in the real world and need to run other things on there too!.
yeah I know it wants to cache in ram . but swap is disk and forcing the OS to use swap when it needs a little ram for anything else makes everything go very much slower than just using the disk normally!
I’ve tried fiddling with my.conf many times .. and a few times briefly got a worthwhile saving and decent performance, but a few days later always found it was again using 2GB ram and the server was again constantly swapping!
is there a source code hack to fix this ?
maybe something like make it see a hardcoded number of my choice for a limit on ram rather than just (I guess) looking at what ram is on the machine and letting those caches grow till they take up take most of it?
I tried even smaller parameters then you suggested but mysql still consuming 425M as same as before. And just convinced myself by what Michael mentioned previously. I rather move to file db rather than using mysql that takes to much memory.
Lyle, you can disable performance schema by putting:
performance_schema = off
under [mysqld] section and then restart the mysql
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