a blog by Marius Gedminas

Profiling with Dozer

Dozer is mostly known for its memory profiling capabilities, but the as-yet unreleased version has more. I've talked about log capturing, now it's time for


This WSGI middleware profiles every request with the cProfile module. To see the profiles, visit a hidden URL /_profiler/showall:

List of profiles

What you see here is heavily tweaked in my fork branch of Dozer; upstream version had no Cost column and didn't vary the background of Time by age (that last bit helps me see clumps of requests).

Here's what an individual profile looks like:

One profile

The call tree nodes can be expanded and collapsed by clicking on the function name. There's a hardcoded limit of 20 nesting levels (upstream had a limit of 15), sadly that appears not to be enough for practical purposes, especially if you start profiling Zope 3 applications...

You can also take a look at the WSGI environment:

WSGI environment expanded

Sadly, nothing about the response is captured by Dozer. I'd've liked to show the Content-Type and perhaps Content-Length in the profile list.

The incantation in development.ini is

use = egg:Dozer#profile
profile_path = /tmp/profiles
next = main

Create an empty directory /tmp/profiles and make sure other users cannot write to it. Dozer stores captured profiles as Python pickles, which are insecure and allow arbitrary command execution.

To enable the profiler, run paster like this:

$ paster serve development.ini -n profile

Bonus feature: call graphs

Dozer also writes a call graph in Graphviz "dot" format in the profile directory. Here's the graph corresponding to the profile you saw earlier, as displayed by the excellent XDot:

Call graph

See the fork where the "hot" red path splits into two?

Call graph, zoomed in

On the left we have Routes deciding to spend 120 ms (70% total time) recompiling its route maps. On the right we have the actual request dispatch. The actual controller action is called a bit further down:

Call graph, zoomed in

Here it is, highlighted. 42 ms (24% total time), almost all of which is spent in SQLAlchemy, loading the model object (a 2515 byte image stored as a blob) from SQLite.

A mystery: pickle errors

When I first tried to play with the Dozer profiler, I was attacked by innumerable exceptions. Some of those were due to a lack of configuration (profile_path) or invalid configuration (directory not existing), or not knowing the right URL (going to /_profiler raised TypeError). I tried to make Dozer's profiler more forgiving or at least produce clearer error messages in my fork branch, e.g. going to /_profiler now displays the profile list.

However some errors were very mysterious: some pickles, written by Dozer itself, could not be unpickled. I added a try/except that put those at the end of the list, so you can see and delete them.

Pickle errors

Does anybody have any clues as to why profile.py might be writing out broken pickles?

Update: as Ben says in the comments, my changes have been accepted upstream. Yay!

Capturing logs with Dozer

Dozer is mostly known for its memory profiling capabilities, but the as-yet unreleased version has more:

Log capturing

This WSGI middleware intercepts logging calls for every request. Here we see a toy Pylons application I've been working on in my spare time. Dozer added an info bar at the top:

Dozer's infobar

When you click on it, you get to see all the log messages produced for this request. I've set SQLAlchemy's loglevel to INFO in my development.ini, which produces:

Dozer's log viewer

(Why on Earth does SQLAlchemy think I want to see the memory address of the Engine object in my log files, I don't know. The parentheses contain argument values for parametrized queries, of which there are none on this page.)

Upstream version displays absolute timestamps (of the YYYY-MM-DD HH:MM:SS.ssssss variety) in the first column; my fork shows deltas in milliseconds. The incantation in development.ini is

use = egg:Dozer#logview
next = main

which makes it disabled by default. To enable, you run paster like this:

$ paster serve development.ini -n logview

(Upstream version lacks the paste entry point for logview; it's in my fork, for which I submitted a pull request weeks ago like a good open-source citizen. Incidentally, patches for stuff I maintain have been known to languish for years in my inbox, so I'm not one to throw stones.)

Next: profiling with Dozer.

Update: Tom Longson blogged about this back in 2008! And his CSS is prettier.

Serving ePub files to Android 1.6

If you click on an ePub download link in the Android browser and get back an error saying "Cannot download. The content type is not supported on the device", then make sure the web server is setting the Content-Type header to "application/epub+zip" and not "application/octet-stream". When you do this, Android will launch FBReaderJ automatically (provided that you have it installed, of course).

N900 connection sharing the hard way

My N900 has a SIM card with a flat-rate 3G data plan. My laptop hasn't. What do I do when I want to use the Internet on my laptop somewhere that doesn't have WiFi? Well, there are many options:

Option 1: N900 as a USB modem

Use the provided USB cable to connect the N900 to the laptop. Choose "PC Suite" mode on the N900 when you get the USB connection menu. The laptop now sees your N900 as a bog-standard USB 3G modem. Use Network Manager to connect to the internet.

Pros: no extra setup required. The N900 and the laptop can both access the Internet at the same time.

Cons: you have to use a USB cable (I hate cables). You cannot ssh into your N900 (and ssh is my primary file transfer protocol between the laptop and the M900).

Option 2: N900 as a Bluetooth DUN modem

Install Bluetooth DUN support from Maemo Extras. Then use it like you would any other phone that has Bluetooth DUN.

Pros: no cables.

Cons: Bluetooth is the worst technology ever. I never had it work reliably. Plus, Network Manager in Ubuntu 10.04 doesn't support Bluetooth DUN (it supports only Bluetooth PAN, as far as I know).

Option 3: N900 as a WiFi access point with Joikuspot

I haven't tried this.

Pros: simple (hopefully), no cables required.

Cons: Joikuspot is non-free. I'm not an absolute zealot, but I will avoid closed-source stuff when open-source alternatives are available.

Option 4: N900 as a WiFi access point with Mobilehotspot

I haven't tried this either.

Pros: it's an open-source app available from Maemo Extras. No cables required.

Cons: requires a non-standard kernel (or so I've heard). Way outside my comfort level.

Option 5: N900 as a WiFi access point with shell scripts

Here's the shell script I run on my N900: share-wifi. It sets up an ad-hoc WiFi network, and starts a DHCP and DNS server (dnsmasq). Sadly, it cannot set up connection sharing (NAT), so I rely on OpenSSH as a SOCKS5 proxy. The whole setup is like this:

  1. You want the latest firmware (PR 1.2) to avoid this bug.
  2. You need to have OpenSSH installed on the N900. Also, setting up key-based authentication makes it more convenient.
  3. The script assumes that you've set up sudo on the N900 so that you can run any command as root.
  4. You need to have wireless-tools installed. It's in the main SSU repository so you should be able to sudo apt-get install it (if it's not preinstalled; I don't remember).
  5. On the N900 run share-wifi in a terminal (optionally passing a WiFi channel number from 1 to 11, in case you need to avoid interference with nearby networks).
  6. On the laptop connect to the new n900 WLAN and run ssh -D 1080 user@n900. You will get a shell session; the SOCKS proxy will be active while it is open.
  7. Reconfigure your laptop to use a SOCKS5 proxy on localhost:1080. For GNOME systems I've a couple of shell scripts: proxy-on and proxy-off. For applications that do not use the GNOME proxy settings (such as Subversion access over SSH), use tsocks.
  8. When done, hit Ctrl-C on the N900 to terminate the sharing script.

Pros: no non-free software or custom kernel required. No cables.

Cons: complicated to set up. No WLAN power savings available for ad-hoc networks, so battery life is extremely poor (~2 hours). But, hey, no cables!