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Using DTrace to Capture Statement Execution Times in MySQL

Introduction

I have recently been engaged with a customer that is evaluating MySQL,
in particular with its Memory (formerly known as Heap) engine, which
stores all database data and metadata in RAM.

I was asked to assist with diagnosing if/whether/where statements were
taking longer than 5 milliseconds to complete. Now, this was being
observed from the viewpoint of the "client" - the client was a
synthetic benchmark run as a Java program. It could be run either on
a separate system or on the same system as the MySQL database, and a
small number of transactions would be measured as taking longer than
5ms.

Now, there is more than one way to skin this cat, and it turns out
that
Jenny
Chen has had a go at putting static probes into MySQL
. For my
(and the customer's) purposes however, we wanted to skip the step of
re-compiling with our own probes, and just use what we can observe via
use of
the
PID provider
.

How Do We Do This?

Well, it is not trivial. However as it turns out, I have seen a bit
of the MySQL code. I also had someone pretty senior from MySQL next
to me, who helped confirm what was happening, as I used some "fishing"
bits of DTrace to watch a mysqld thread call functions as we ran the
"mysql" client and entered simple statements.

This allowed me to narrow down on a couple of vio_\*() routines, and to
build some pseudo-code to describe the call flow around reception of
requests from a client, processing of the statement, then return of a
result to the client.

It is not as simple as looking for the entry and return of a single
function, because I wanted to capture the full time from when the
mysqld thread returns from a read(2) indicating a request has arrived
from a client through to the point where the same thread has
completed a write(2) to send the response back. This is the broadest
definition of "response time measure at the server".

The Result

The result of all of our measurements showed that there were no
statements from the synthetic benchmark that took longer than 5 ms to
complete. Here is an example of the output of my DTrace capture (a
histogram of microseconds):

bash-3.00# ./request.d -p `pgrep mysqld`
\^C
value ------------- Distribution ------------- count< 0 | 0
0 |@@@@@@@@@@@@@@@@@@@@@ 10691
500 |@@@@@@@@@@@@@@@@@ 8677
1000 |@ 680
1500 | 31
2000 | 0

The Script

Feel free to use the
DTrace script
for your own purposes. It should work on MySQL 5.0 and
5.1.

The Upshot - Observability in the JVM

There is a nagging question remaining - why was the Java client seeing
some requests run longer than 5 ms?.

There are three possible answers I can think of:


  1. There is latency involved in transmitting the requests and responses between the client and server (e.g. network packet processing).
  2. The thread inside the JVM was being pre-empted (thrown off the CPU) between taking its time measurements.
  3. The measurements (taken using
    System.nanoTime()) are not reliable.

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