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Performance Victory5/10/2005
I can confidently declare victory in the battle with the performance of
cmScribe's permissions code. A page hit that was taking "about a
minute" every time (the logging that enabled me to determine exactly
how long things are taking wasn't added until later) is now taking 14
seconds on the first hit and is no different than other pages within
the site on subsequent hits.
While working on this I re-learnt another obvious lesson about coding
for performance, which can be summed up as: DON'T trust your instincts, MEASURE. This doesn't
necessarily require complicated profiling tools (although I'm sure if
you know how to use them they can be very useful). All I did was add
code to log every permission lookup and every database hit to a file.
But running a few "grep | wc" operations across the resulting log files
gave me exceptionally useful information about which tables were being
accessed excessively, and proved my gut instincts to be sorely lacking.
My initial feeling was that my best bet would be to try to avoid
excessive hits to two tables, which we'll call TC and V. I spent a
while working on TC and was disappointed to find that I'd only improved
from 43 to 42 seconds. That was when I fired up grep and produced a
little shell script which ran over my initial log giving output like
this:
P: 9115
TC: 735
V: 4408
Turned out I was right about V being critical, entirely wrong that TC
mattered at all (I'd reduced it to 234, which naturally made very
little difference), and horrifyingly wrong to have entirely ignored P
which was the worst offender by an order of magnitude.
The really scary thing about these numbers is that V contains 33
records and never changes at all (except with new builds of the
software) while P contains about 250 and changes rarely (only on
certain administrative actions).
Armed with this knowledge it was an absolute no-brainer to bring the
entire contents of V and P into memory once and leave them there
thereafter (with some code to re-fetch the contents of P when those
administrative actions happen).
Performance improved by a factor of four, DB hits reduced by a factor
of more than ten (from nearly 20,000 to under 1,700), and all without
any need to fundamentally change the architecture of the system.
But I never would have got there if I'd only gone with my instincts
about what could be improved. It was only by producing directly
measurable information about what was really going on that I was able
to spot the evil 9,000 hit table :)
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Programming and Performance5/8/2005
The approach I take to performance issues while coding is that
performance issues should be in the back of your mind at all times. Not
to ignore them, but also to resist the temptation to focus on
performance too much during the design and initial implementation of a
feature, and planning to revisit the issue if performance problems
become apparent later.
This philosophy has both strengths and weaknesses and recent events have showcased both of these.
cmScribe uses a complex and flexible fine-grained permissioning
mechanism where permissions can be granted to all kinds of actions on
all kinds of objects. Having certain permissions can cause others to be
granted implicitly, and the rules for this kind of implication can be
any arbitrary C# code. Since the permissions are so fine-grained, any
given page hit can require a large number of permissions to be
evaluated. Furthermore, the implication rules mean that evaluating one
permission may require a number of others to be evaluated as well.
The system is so complex, in fact, that I struggled quite a lot during
the initial design process to come up with a way of meeting all the
requirements at all. (Is it over-engineered? I don't know. I do know
that after using it for a year there's only one feature I'd have cut,
and that's never been used and doesn't add any complexity) The first
and biggest advantage of keeping performance issues on the backburner
is that if I'd had to juggle performance along with all the other
constraints I was trying to meet, I don't know whether I'd have been
able to produce a working system in the first place. In this case,
deferring performance for later may have made the difference between
impossible and possible.
Since then I've had to revisit this code for performance reasons on two
or three separate occasions. You could look at this as a disadvantage
of the approach I took: surely if performance had been designed in from
the very beginning then I wouldn't have had to repeatedly fix
performance problems later. But you can also look at it as a strength:
the code worked adequately to start with without spending the time on
performance. Later, as more demanding scenarios came up, it was
possible to fix it without too much trouble to again perform
adequately, by a combination of caching frequently-used information in
memory, tweaking the order of operations to make the common cases use
less steps, and micro-optimizing the individual steps to eliminate
avoidable database hits and other expensive operations. I'm in the
middle of an iteration of that process right now, and I'm entirely
confident that I can have it performing adequately again shortly.
The weakness of the approach, however, is that an architecture designed
without considering performance (since I was struggling so much with
all the other issues, performance was probably further back in my mind
even than usual) has turned out to have some performance bottlenecks
that simply can't be removed without changing the architecture itself.
There are situations where it's possible to know based on fixed
information that there's no way a user could possibly have a particular
permission, but that fixed information isn't available within the
architecture, so the code will still chase down a number of dead ends
before it arrives at the answer. And there's no way to make that
information available with little caching tweaks and
micro-optimizations. It needs a whole new structure.
For now, I can continue to tweak the heck out of the existing
architecture and I'm confident it will perform adequately for quite
some time. Which leads to the final advantage - even when you do reach
the point where there's nothing to be done but throw out the whole
thing and start over with performance at the very front of your mind,
the experience gained from the first attempt will be invaluable in
designing the system the right way. Every tweak I make to the existing
code will be designed into the next version from day one.
Sounds a lot better than being stuck a year ago unable to write the
thing at all because I couldn't get my head around how to make it fast, doesn't it? :)
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Taking advantage4/25/2005
Thanks to everyone who's emailed or commented supportively. Jeff in
particular, thank you for a much needed laugh, and I too hope that what
I actually have is Nullable<Cancer>. Also Jeroen and Mark for the
thoughtful emails, Jim for the comment on his own blog, and everyone at
work and everyone I know in person for their thoughts and prayers (I
may not believe in prayer personally, but I appreciate the thought from
people who do).
I arrived at work this morning to find that lots of people were sick with
colds, headaches, etc - and that's not including the people who were out sick. The conversation went something like...
Coworker 1: "We're all a bunch of invalids today..."
Me: "Well, I have cancer -- I win!"
Coworker 2: "My husband's sick and he's also having a colonoscopy"
Me: "I have cancer -- I still win!"
Coworker 2: "Fair enough"
Normally when I or members of the family are sick I'll struggle through
and work from home, or sometimes feel guilty and leave Janene to suffer
while I go into the office because there's stuff that simply needs me
to do it. But right now even when I'm in the office I can't really
focus, and besides, if there's anything in life that entitles you to
take advantage and take a little bit of a break to recuperate, it's
having cancer.
So for the rest of this week I've pledged that I'm not going to feel
obligated to get any work done. That's not to say I won't do anything
that will benefit my work, but I'll focus on stuff I want to do with
long-term benefits, rather than the never-ending stream of kludgy
customer-specific fixes that drive my stress levels through the roof at
the best of times.
(By the way, this means among other things that I won't be receiving
any @netreach.com email - if you want to reach me, use the gmail
address at the bottom of every page of my site)
So here's a list of projects, work-related and not, that I intend to attempt over the next few days:
- Get japitools handling some JDK5.0 features. I've started this
already - I have a version of japicompat that can theoretically cope
with a lot of the "interim" japi file spec version 0.9.7 that supports
some, but not all, of the 5.0 features. Unfortunately I don't have any
way of creating japi files in that format: Jeroen, if you're
reading this, do you have any tips on how to get the necessary metadata
out of the class files?
- Get nrdo integrated into the new Visual Studio 2005 beta in the
cleanest possible way. This means using List<T> everywhere,
nullable types everywhere (an act of faith that these will be adequate
by final release) and somehow hooking it into the build system in such
a way that, hopefully, we don't require two separate extra project
files and to rebuild the whole thing twice just to pick up the
generated code.
- Produce
a release of NRobot to include the new security code, and announce it
in enough places that perhaps some people will try producing robot
implementations...
- Watch all three LotR extended editions, especially RotK which I've never seen even the standard edition of.
- Continue to push the nullable type issue with Microsoft any way I can find.
- Learn as much as possible about Visual Studio 2005 and how the
migration will impact cmScribe. I think that actually doing a migration
will take longer than the few days I have, but hopefully I can at least
figure out what the biggest issues will be.
- Catch up on DVR'd TV shows that I haven't watched yet.
- Oh yeah, recover from the surgery...
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Sleep Deprivation2/17/2005
I've noticed a strange symptom when I get sleep deprived: I start
getting silly in stuff I type. It's curious because it really does seem
to be limited to when I'm expressing myself textually - I'm perfectly
normal (at least as much as I ever am) in person or on the phone. Last
night I was up until 4am finishing some stuff for work and up again at
6:30am. This morning I was writing one of my ubiquitous TODO lists
trying to pin down the steps to implement a particular cmScribe
feature. The approach I had in mind at the time left some user actions
open that could cause problems, and I couldn't figure out how to
implement restrictions that would prevent them (doesn't seem silly so
far, does it?). After a little brainstorming I had a "Eureka" moment
when I realized that a different approach would avoid the problematic
scenarios altogether. I immediately noted the revelation in my TODO
file thusly:
- IKNOWIKNOWIKNOWIKNOWIKNOWIKNOW!
After a moment's pause I amended the note:
- <twinkle>IKNOWIKNOWIKNOWIKNOWIKNOWIKNOW!</twinkle>
You probably need to be a parent of young children to get the
reference, I'm afraid. In the end after a little research and
discussion it was determined that actually, the feature that led to all
this hilarity didn't actually have any practical uses, so my great
insight was entirely wasted. Awww pickles.
On another similarly sleep-deprived occasion I sent an email to a
client to report finishing a troublesome feature. The email began "It's
alive! IITT'SSS ALLIIIIIIVVVEEEE!!! Ahem."
There have been other examples of the same thing but I can't remember
what they were. Maybe I'd be able to remember if I'd had more sleep ;)
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API design, part 22/11/2005
When faced with the task of designing APIs for a problem that has a lot
of inherent complexity, an essential first step is to design exactly
how and where to hide that complexity. I was recently faced
with the need to redesign an API that I'd created before I learnt this
lesson, and I think I did a good job of it - the complexity is almost
entirely hidden from users on both "producer" and "consumer" sides of
the API.
However, in the process I've learnt another essential, if (in
retrospect) obvious lesson: Just because you've figured out how to hide
the complexity doesn't make the complexity go away. Implementing the
code that actually does the hiding is still complex, probably
even more so due to the need to provide the illusion of simplicity.
Forget this at your peril!
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