Posted on October 28th, 2011 in programming, Python | 3 Comments »
As I don’t develop C# application at work anymore, I was finally allowed to switch from Windows to Linux. So I grabbed the 64-bit version of Mint Debian Xfce, installed it and moved my Thunderbird profile and my Python projects. After installing new Python virtual environment I launched the tests for our biggest project using Twisted. Tests passed, but it seemed to me that they run way too long. So I launched Windows once again and compared the time. Under Windows XP tests run for about 40 seconds and under Linux – for over 200 seconds. So I wondered how the hell was this possible, Linux can’t be 5 times slower than Windows.
I run the tests under Linux and checked the system performance stats. The CPU cores utilization was about 12%. After further investigation I discovered that during tests over 40% of the time CPU was waiting for the disk. So I installed the iostat tool, which showed me that 95% of the IO operations were made by the jbd2 process.
When installing Mint, I decided to give the ext4 partition a chance, as I haven’t used it before. The jdb2 process is updating the ext4 journal. As far as I know ext3 has a journal too, but I didn’t noticed any problems with it.
After wasting some time trying out some random solutions I googled for, I found out that adding the nobarrier flag for my ext4 partition solves my performance problem. It seems the MySQL is using the fsync extensively, which can lead on ext4 to some serious performance problems. Disabling the barriers fixed the problem, but I wasn’t happy with it, because this option may result in hard filesystem crashes in case of power failure.
And so to investigate the problem I launched the Mint Live CD, shrunk my root partition and created a new 30GB partition for my MySQL data. After that I ran my integration tests while changing the type of this new MySQL partition. Here are the results:
|Partition type||Time (in seconds)|
* both the testing application and the database were on the same ext4 partition
Well, as any of us, I want my applications to respond quickly to our users. And sometimes the database can be a bottleneck, so I very carefully design the table schema and create the indexes. As I discovered, the performance downgrade can come from an unexpected direction. In my example, it was a partiotion type, so learn from my mistake and DO NOT CREATE EXT4 PARTITION FOR MYSQL DATA :). If you, for some reasons, have to use ext4 for that, you can disable the barriers, but be aware of the potential data loss in case of power failure. Right now ext3 seems to me to be the the best choice – despite having a journal, it is faster than ext2 (in fact it’s the fastest partiotion type for MySQL).
The ext4 doesn’t seem to be ready right now. The interesting part about it was that two ext4 partitions on the same disk (one for the system and testing code and the second for the database) were faster than a single ext4 partition – it seems that MySQL fsyncs slowered down the whole system.
Luckily there are a lot of performance tracking tools (like top or iostat) that monitor the CPU/memory/IO/network usage. This takes some time, but helps to track down the problem.
And the most important thing – at the end, the Linux proved to be faster that Windows, so I don’t have to go back to my XP installation. Phew! :)