Re: Berkeley DB license

I know people are still reviewing the SDB implementation for PostgreSQL,
but I was thinking about it today.

This is the first time I realized how efficient our current system is. 
We have shared buffers that are mapped into the address space of each
backend.  When a table is sequentially scanned, buffers are loaded into
that area and the backend accesses that 8k straight out of memory.  If I
remember the optimizations I added, much of that access uses inlined
functions (macros) meaning the buffers are scanned at amazing speeds.  I
know inlining a few of those functions gained a 10% speedup.

I wonder how SDB performs such file scans.  Of course, the real trick is
getting those buffers loaded faster.  For sequential scans, the kernel
prefetch does a good job, but index scans that hit many tuples have
problems, I am sure.  ISAM helps in this regard, but I don't see that
SDB has it.

There is also the Linux problem of preventing read-ahead after an
seek(), while the BSD/HP kernels prevent prefetch only when prefetch
blocks remain unused.

And there is the problem of cache wiping, where a large sequential scan
removes all other cached blocks from the buffer.  I don't know a way to
prevent that one, though we could have large sequential scans reuse
their own buffer, rather than grabbing the oldest buffer.

-- 
  Bruce Momjian                        |  http://www.op.net/~candle
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