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[John Arbash Meinel <john@xxxxxxxxxxxxxxxxx>]

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I've been digging through some bzr-on-launchpad performance problems,
and I've come across something interesting. I was wondering if anyone
hand feedback that could help me.

Here is the bug I was tracking into:
https://bugs.launchpad.net/launchpad/+bug/740759

Basically what I've found is that twisted.python.failure.Failure is
instantiated when a Deferred raises an Exception. And Failure scales
O(state).

Specifically, it walks all the stack frames and traceback frames, and
copies all of the global and locals. When the deferred thinks it is done
with the traceback, it then walks all of those frames again, and calls
'safe_repr' on everything. So that you still have hints at all the
state, but you don't actually reference the original objects.

For a hint as to how expensive this is. When creating a new branch, we
call 'initialize_...'. Doing this in various permutations:

 30ms	Directly on my VM
 450ms	Using codehosting on my VM
 250ms  Codehosting with Failure.cleanFailure hacked to not call
        safe_repr, but keep references to the original objects.
 183ms	Codehosting with Failure.__init__ hacked to always set
	'tb = None'

And of the 183ms, 147ms was spent in XMLRPC calls. So at least "CPU
time" is back down to the 30ms in bzrlib.

Now, I can understand that Failure is meant to provide useful feedback
about the original state when an exception doesn't get handled.

But in bzrlib, and certainly in Launchpad exceptions are often used for
signaling. They certainly don't signal fatal conditions on every exception.

I don't think it is very tasteful to hack Twisted to supress the 'tb'
parameter. Certainly the code isn't written to be modular in that
respect. All the walking is done directly in the __init__ function, so
it can't be simply overridden.

Any hints are welcome.

John
=:->
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