perfmetrics

The perfmetrics package provides a simple way to add software performance metrics to Python libraries and applications. Use perfmetrics to find the true bottlenecks in a production application.

The perfmetrics package is a client of the Statsd daemon by Etsy, which is in turn a client of Graphite (specifically, the Carbon daemon). Because the perfmetrics package sends UDP packets to Statsd, perfmetrics adds no I/O delays to applications and little CPU overhead. It can work equally well in threaded (synchronous) or event-driven (asynchronous) software.

Complete documentation is hosted at https://perfmetrics.readthedocs.io

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Usage

Use the @metric and @metricmethod decorators to wrap functions and methods that should send timing and call statistics to Statsd. Add the decorators to any function or method that could be a bottleneck, including library functions.

Caution

These decorators are generic and cause the actual function signature to be lost, replaced with *args, **kwargs. This can break certain types of introspection, including zope.interface validation. As a workaround, setting the environment variable PERFMETRICS_DISABLE_DECORATOR before importing perfmetrics or code that uses it will cause @perfmetrics.metric, @perfmetrics.metricmethod, @perfmetrics.Metric(...) and @perfmetrics.MetricMod(...) to return the original function unchanged.

Sample:

from perfmetrics import metric
from perfmetrics import metricmethod

@metric
def myfunction():
    """Do something that might be expensive"""

class MyClass(object):
    @metricmethod
    def mymethod(self):
        """Do some other possibly expensive thing"""

Next, tell perfmetrics how to connect to Statsd. (Until you do, the decorators have no effect.) Ideally, either your application should read the Statsd URI from a configuration file at startup time, or you should set the STATSD_URI environment variable. The example below uses a hard-coded URI:

from perfmetrics import set_statsd_client
set_statsd_client('statsd://localhost:8125')

for i in xrange(1000):
    myfunction()
    MyClass().mymethod()

If you run that code, it will fire 2000 UDP packets at port 8125. However, unless you have already installed Graphite and Statsd, all of those packets will be ignored and dropped. Dropping is a good thing: you don’t want your production application to fail or slow down just because your performance monitoring system is stopped or not working.

Install Graphite and Statsd to receive and graph the metrics. One good way to install them is the graphite_buildout example at github, which installs Graphite and Statsd in a custom location without root access.

Pyramid and WSGI

If you have a Pyramid app, you can set the statsd_uri for each request by including perfmetrics in your configuration:

config = Configuration(...)
config.include('perfmetrics')

Also add a statsd_uri setting such as statsd://localhost:8125. Once configured, the perfmetrics tween will set up a Statsd client for the duration of each request. This is especially useful if you run multiple apps in one Python interpreter and you want a different statsd_uri for each app.

Similar functionality exists for WSGI apps. Add the app to your Paste Deploy pipeline:

[statsd]
use = egg:perfmetrics#statsd
statsd_uri = statsd://localhost:8125

[pipeline:main]
pipeline =
    statsd
    egg:myapp#myentrypoint

Threading

While most programs send metrics from any thread to a single global Statsd server, some programs need to use a different Statsd server for each thread. If you only need a global Statsd server, use the set_statsd_client function at application startup. If you need to use a different Statsd server for each thread, use the statsd_client_stack object in each thread. Use the push, pop, and clear methods.

Graphite Tips

Graphite stores each metric as a time series with multiple resolutions. The sample graphite_buildout stores 10 second resolution for 48 hours, 1 hour resolution for 31 days, and 1 day resolution for 5 years. To produce a coarse grained value from a fine grained value, Graphite computes the mean value (average) for each time span.

Because Graphite computes mean values implicitly, the most sensible way to treat counters in Graphite is as a “hits per second” value. That way, a graph can produce correct results no matter which resolution level it uses.

Treating counters as hits per second has unfortunate consequences, however. If some metric sees a 1000 hit spike in one second, then falls to zero for at least 9 seconds, the Graphite chart for that metric will show a spike of 100, not 1000, since Graphite receives metrics every 10 seconds and the spike looks to Graphite like 100 hits per second over a 10 second period.

If you want your graph to show 1000 hits rather than 100 hits per second, apply the Graphite hitcount() function, using a resolution of 10 seconds or more. The hitcount function converts per-second values to approximate raw hit counts. Be sure to provide a resolution value large enough to be represented by at least one pixel width on the resulting graph, otherwise Graphite will compute averages of hit counts and produce a confusing graph.

It usually makes sense to treat null values in Graphite as zero, though that is not the default; by default, Graphite draws nothing for null values. You can turn on that option for each graph.

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