On Good Instrumentation

In so many cases troubleshooting applications I keep thinking how much more efficient things could be going if only there would be a good instrumentation available. Most of applications out there have very little code to help understand what is going on and if it is there it is frequently looking at some metrics which are not very helpful.

If you look at the system from bird eye view – system needs to process transactions and you want it to successfully complete large number of transactions it gets (this is what called availability) and we want it to serve them with certain response time, which is what is called performance. There could be many variables in environment which change – load, number of concurrent users, database, the way users use the system but in the nutshell all what you really care is having predictable response time within certain range. So if we care about response time – this is exactly what our instrumentation should measure

Response Time Summary We want to understand where exactly response time comes from. For example if we define transaction as the time it took to generate HTML page we want to understand how much time was spent waiting on the database, memcache, other external services, as well as how much CPU time it consumed.

Now what is important we need this information for individual transactions. It may be every transaction which is best and easily achievable for small-medium systems or at least for large enough sample. It is very important this information is available for individual transactions not the average. Average is useless because 100 transactions taking 1 sec and 99 transactions having 1ms and 1 taking 99.1 sec will have the same average while for sake of performance analyzes these are completely different. When you have transaction sample make sure it contains fair population of transaction – getting only transactions which are slow is not helpful as we might want to compare them to the fast transactions to understand why they are slow.

What kind of components do you need to have in response time summary – all components which are significant enough. If your instrumentation has 95% of response time unaccounted for it is useless. You also want blocks not to mix apples with oranges. For example “mysql and memcache” block would not be helpful. Even further I would prefer to split “mysql time” in the “connect time” and “query time” as there are situations when one but not other would be affected.

In is important for response time summary stored in the logs which are easy to query so you can analyze data in a lot of various ways. Sometimes you may find the response time is impacted by queries from certain user, in others it may be attributed to different application/web server.

The goal for Response Time Summary is to quickly point direction where problem happens. Whenever you have spike in response time or it is bad response time for certain kind of request you can quickly understand where does it come from ? Is it wasted CPU time Slow response from MySQL or Memcache.

I also like to see numbers of calls stored together with attributed response time. For example I’d like to see number of mysql calls in addition to MySQL response time. This helps to understand if it is the issue with number of queries or their performance. If I see 2 queries taking 30 seconds it is clearly slow queries. If it is 10.000 queries executed and total response time is 4 sec I know it is pretty much as good as it gets with standard Ethernet network and finding a ways to reduce number of queries is going to be more helpful.

The Glue Our applications involve multiple layers and typically higher layer can only report response time it took to call lower layer, but not the reason for that response time. For example we can report time it took to execute SQL query from PHP application side, but we can’t say why it has taken so long. Was it row level lock ? waiting on disk IO or was it simply question of burning a lot of CPU. On the other hand this information may be available in the instrumentation stats from that lower layer – for example in MySQL Query Log. What is important is however to be able to connect the data from these logs – glue them together. The easy way to do it is to provide an unique identifier to all requests and put it in the logs with request of the lower levels. With mySQL the simple way to do it is to put it in the comments for queries you execute.

Optional Tracing The information in lower layers logs is very helpful however it typically have two problems. First not every layer has good logs. For example if you’re running memcached you probably do not have the logs detailing all requests and their response time. Second – the lower layer may only know response time from its vantage point, which in many cases does not include network communication time which can be very important.

Tracing should be optional and normally applied to the small sample of requests, though it needs to be detailed. Typically you would include the calls to the lower level services together with timestamp, the response summary with timestamp again. The information about request has to be complete enough to identify target action and response completely. For example if I’m speaking about memcache I’d like to know which server:port request was issued to which key was requested, and on response I’d like to know if it was hit, miss or error.

The way I use it may be as follows. I see the increased response time for given kind of request. I see response time is coming from MySQL. I check the number of MySQL Queries and it is 5x when it usually is for this kind of request. Looking at memcache stats I can see high number of misses. Looking at some available traces shows the server memcache01 has very high miss ratio. Checking what is going on with memcache01 shows it just was restarted (and hence has almost empty cache). This is important example as it shows your increased response time from MySQL may not have anything to do with MySQL itself but you would not know unless you’re capturing the right data.

If you’re looking for nice example framework for instrumentation, check out instrumentation for PHP – It has everything mentioned by tracing which is trivial to add.

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