Valgrind

Valgrind

Valgrind is useful for performing dynamic analysis. Valgrind tools can automatically detect many memory management and threading bugs, and profile Peloton in detail.

Memory Leaks

Here's the command for using memcheck for detecting common memory errors.

valgrind --trace-children=yes --track-origins=yes peloton -D ./data

valgrind --trace-children=yes --track-origins=yes ./test/catalog_test

Profiling

Here's the command for using callgrind for profiling Peloton.

    valgrind --tool=callgrind --trace-children=yes ./src/peloton -D data &> /dev/null &

    pg_ctl -D ./data stop

You can use kcachegrind for viewing the collected profile files (i.e. the callgrind.out.* files).

    kcachegrind

Profiling with Perf

The Linux kernel comes with a performance analysis tool called perf that can even be used on production servers to analyze the call stack of a server, or a process. This is useful to find the bottlenecks of a query processing or simply of a server running and improve those.

It is usually found in the package "perf", at least it is the case of RHEL, CentOS and ArchLinux.

/proc/sys/kernel/perf_event_paranoid can be used to restrict access to the performance counters.

 2, allow only user-space measurements
 1, allow both kernel and user measurements (default)
 0, allow access to CPU-specific data
-1, no paranoid at all

Record information

## Profile system as long as needed, can be cancelled with Ctrl-C  
perf record -a -g

## Profile system for a given period of time  
perf record -a -g -s sleep $TIME_IN_SECONDS

## For the duration of a command  
perf record -a -g -s -- $COMMAND

Different things can be recorded:

-a to profile the whole system
-p $PID to profile only the given PID
-u $USER to profile only the given user

Reports are saved by default in $HOME/perf.data. Old reports are renamed as $HOME/perf.data.old.

## Real-time measurement is done by perf top, like system-wide profiling  
perf top

## Profiling without accumulating stats
perf top -z

Viewing the records

## View profile recorded
perf report -n

## With a graph
perf report -g

Debugging with Strace

## Run peloton under strace
strace -o trace.server -f -t peloton -D /path/to/data

## Run psql under strace
strace -o trace.client -f -t psql postgres

The first flag is -f, which tells strace to follow the forked processes. The -o flag sends all the output to a file. We can also use the -ff flag, that sends the output corresponding to each forked process to a separate file. Very handy, that. Finally, we add a -t flag, which prepends each line with a timestamp.