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From Greg Ames <ames.g...@gmail.com>
Subject Re: Fixing Ranges
Date Thu, 25 Aug 2011 16:45:26 GMT
On Thu, Aug 25, 2011 at 10:25 AM, Jim Jagielski <jim@jagunet.com> wrote:

> Now that the memory utilz is being fixed, we need to determine
> what sort of usage we want to allow… I'm guessing that people
> are ok with http://trac.tools.ietf.org/wg/httpbis/trac/ticket/311
> as the guiding spec?


I'm no longer convinced that we *need* to merge/optimize the Range header.
Stefan's fix to the brigade plumbing has converted O(N^2) memory + CPU use
into something scalable.

Think about the effect of this attack on the CPU cache.  My laptop has a
Core i7 with a 64kB L1 data cache.  The cache lines are 64 bytes, so I have
1024 of them per core.  The Range header I'm using has ~1300 byterange
specifiers.  apr_brigade_partition in this case will fragment the brigade
into ~1300 buckets with length 1 plus one or two for the remainder of the
file.  Each of those has to be read into the L1 cache somewhere as
apr_bucket_split walks thru the bucket structures.  That fills the cache
with junk and throws out the useful information, like the request_req,
conn_req, the filter structures, and most of the stack.  So not only will
the apr_brigade_partition calls perform like a dog but the mainline code
will suffer too.

Greg

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