binary protocol notes from the facebook hackathon

[Steve Grimm]

On 7/11/07 12:45 AM, "marc at corky.net" <marc at corky.net> wrote:
> While network byte ordering (Big Endian) is traditionally the 'right'
> thing to do (or the default thing to do), in most cases it's a minor
> performance hit due to constant swapping.  Since we're implementing a
> binary protocol specifically to avoid/minimize minor performance hits
> and since this is a brand new protocol I would recommend to keep all
> values as Little Endian because:

Defining a protocol that uses something other than network byte order gives
me the willies. But I'm not sure I can argue with your reasoning on anything
more than aesthetic grounds; swapping bytes around unnecessarily is exactly
the sort of thing we're trying to avoid.

I wonder just what the overhead of byte swapping will be, though, compared
to all the other work that has to happen to handle a request. It may be a
small enough expense to disappear into the statistical noise. If we'd be
saving .001% of the CPU time by avoiding swapping, I'd say forget it, not
worth the confusion.  I've never actually measured the cost of using network
byte order in any of my network apps, though, so I honestly have no idea.

If it's a large overhead, maybe we can make endianness depend on the magic
number at the start of the command. It is annoying to support both, granted,
but that would potentially allow a client to be configured to send requests
and receive responses in whatever format is most efficient for a particular
server. (Or vice versa, I suppose.) Or would the overhead of having to
support both styles eat all the cycles you'd save by not swapping bytes? I'm
not sure.

I suspect that the Sun folks would appreciate not having to flip bits back
to network byte order on their SPARC boxes, but I agree with you that Intel
and AMD totally dominate memcached's target audience at the moment.
Supporting both orders from the start would insulate us from changes in that
market reality, though maybe that's far enough off that it's better to just
rev the protocol if/when the need arises.

-Steve