Re: L2 cache vs. faster processor
- From: Mladen Gogala <mladen@xxxxxxxxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Wed, 14 Jul 2004 16:13:25 -0400
Definitely L2 cache. My numbers may be a little bit out of date, but the story
remains
the same. L2 cache is a sequential 15ns portion of memory which caches your
RAM. L1 cache is
very small, typically 64k or 128k set associative RAM. There is also something
called TLB
which is a fully associative buffer with 64 locations. The rest of your RAM is
50ns SDRAM,
with "burst mode". So, what happens in a VM system when program requires the
value of the
variable in the address X? CPU has to resolve the location X. In order to
resolve the location
X, it will have to go to memory and fetch the page table first. Then it will
look through
the page table, find the address and again go to memory and get the page
itself. That means
that resolution of each address needs 2 accesses to the main memory, which is
equally fast,
regardless of the CPU. If no L2 cache is present, efficiency of L1 cache is
approximately
25%. L1 cache is invalidated as soon as someone issues a jump, like, let's say.
a call to
a subroutine (fashionable expression is "method"). L2 cache only cache physical
pages, in
order to help the execution. Together with L1, the typical efficiency is around
70%-90%,
depending on the size of L2 cache. One other thing to know is that the speed of
waiting is
equal on both 3GHZ chip and 2GHZ chip. 3GHZ chip waiting 3 times for 100 nsec
access to
RAM will spend 300nsec of its time waiting, while 2GHZ chip with L2 cache will
be working....
The only programs that will be faster on 3GHZ chip without L2 cache are the
ones that are
written with locality of reference in mind, and which don't do nasty things
like context
switches (every system call, like I/O) of call subroutines. Ultimately, the
answer depends
on what you want to do. If you are a rocket scientist designing ship for the
mission to
Mars with Carrie Ann Moss, you'll probably want 3GHZ without L2 cache. For
oracle, on
the other hand, you do want L2 cache.
On 07/14/2004 03:23:16 PM, Terry Sutton wrote:
> A client is ordering hardware for an Oracle on Linux system, and has to
> choose between 2.8 GHz Xeon processors with 1MB L2 cache and 3.06 GHz Xeon
> processors (no L2 cache). Does anyone have any recommendations on the value
> of L2 cache vs. a faster processor?
> --Terry
>
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--
Mladen Gogala
Oracle DBA
Wang Trading LLC
Tel: (203) 956-6826
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