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Re: latest svn rev 11720 become cpu hungry

On Tue, Oct 02, 2007 at 05:36:02PM +0800, Li-Hui Zhou wrote:
> I've noticed the bugfix and yeah, it's been a LOT better.
> Old bug? Why it not been triggered until recent svn?

Short version: There was an optimization that worked around it, but
that optimization no longer applied.

Long version: The format of a "router list," as we parsed it used to
     (ExtraInfo | RouterDesc)*
In other words, it had any number of extrainfo and routerdesc
documents, possibly scrambled up.

The old code did:
   - If we start with the word extra-info, it's an extra-info and
     we're done.
   - If we start with the word router, it's a routerdesc and we're
   - Otherwise, scan for the first instance of the word router and
     scan for the first instance of the word extra-info.  Whichever
     comes first is the next document.

This was fine until we added annotations around r11680.  The format
    (Annotation* (ExtraInfo | RouterDesc))*
Now, the first two cases no longer applied when there were
annotations, since the point where we were looking never started with
the word router or the word extra-info, so we always did case 3.
But in a list like the cached-descriptors list that has no extra-info
documents, we wound up scanning the entire list looking for an
extra-info that never existed, and we did this scan for every router
in the cache.  That's O(n^2) in cache size, and that's no good.

For the fix, see the patch. :)

Nick Mathewson

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