From: Kevin Lewis
Date: October 6 2008 5:07pm
Subject: Understanding RefCounts in Falcon
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Falcon Team,

In looking through the Falcon code, one can see heavy use of reference 
counters; calls to an object's addRef() or release() functions.  These 
are manual reference counters and they always do an interlocked 
increment or decrement on the reference count.

A smart pointer is an automatic way of encapsulating this kind of 
reference counting, but it is automatically done every time a copy is 
made of a pointer.

The advantage of targeted reference counting is performance.
The advantage of smart pointers is stability.

We have been dealing with unbalanced reference counters in the 
DeferredIndex object lately, but have dealt with it before in BDBs and 
Record objects, most notably.  If a reference is added but not deleted, 
you get a memory leak.  The opposite causes a crash, when an object 
disappears prematurely.

The performance advantage of targeted reference counters is real. 
Today, for example, I saw a performance report from the Intel team in 
China indicating that the Record::addRef is one of the top 2 functions 
which increased CPU cycles as a sysbench test increased from 10 to 20 Mb.

If Record::addRef() used smart pointers, the performance drop would be 
measurable and multi-core scalability would suffer even worse.  Since 
one of our goals is to scale well on multi-core systems.  It makes sense 
for us to be careful about things which would slow us down.

Instead of sweeping reforms like smart pointers which add restrictions 
to performance and scalability, I think we should take the time to 
understand the correct use of targeted reference counters, and use them 
wisely.

Not only that, we should look for opportunities to group reference 
counters into larger scopes.  Granted, those opportunities are difficult 
and risky.

Kevin Lewis
Falcon Team Lead