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SubjectRe: X15 alpha release: as fast as TUX but in user space
On Sun, 29 Apr 2001, Fabio Riccardi wrote:

> I can disable header caching and see what happens, I'll add an option
> for this in the next X15 release.

heh, well to be honest, i'd put the (permanent) caching of the Date header
into the very slimy, benchmark-only trick category. (right up there
alongside running the HTTP and TCP stacks inside the NIC interrupt
handler, which folks have done to get even better scores.)

> Nevertheless I don't know how much this is interesting in real life,
> since on the internet most static pages are cached on proxies. I agree
> that the RFC asks for a date for the original response, but once the
> response is cached what does this date mean?

the Date is always the time the response was generated on the origin
server. (there are exceptions for clockless servers, but such servers
have other limitations you wouldn't want -- notably they MUST NOT generate
Last-Modified.)

one example oddity you might experience with a non-increasing Date
surrounds Last-Modified and Date, see section 13.3.3. note that the rfc
indicates that if Last-Modified is less than 60 seconds earlier than Date
then Last-Modified is only a weak validator rather than a strong
validator. this would complicate range requests -- because weak
validators can't be used with range requests. if your server never
updates the Date after the first time it serves an object then you'd
potentially never get out of this 60 second window.

(strong validators are guaranteed to change whenever the object changes...
and Last-Modified isn't strong until some time has passed -- consider NFS
mounted docroots, clock skew in the origin network, multiple file updates
within a second, etc.)

there are a bunch of other things that Date is used for, all of them are
related to caching heuristics and rules.

in theory you could claim that you're implementing a cache server rather
than an origin server... i dunno what the SPEC committee will think when
you try to submit results though :)

so way back when sendfile() was being created i brought up the Date issue
and pointed out that we needed more than a single "void *, size_t" to take
care of headers. eventually this discussion lead creation of TCP_CORK --
so that a http server could use writev() to send a two element iov for the
headers -- one element with everything that doesn't need to change, the
next element with the Date.

i also kind of expected the high performance servers to rebuild a Date:
header every second for all of its threads to use. (of course it's not
that simple, you really want to keep a circular list of N dates... and
just assume that after N seconds no thread could still be accessing an old
Date.)

is this too slow for some reason? (does it play well with zero-copy?)

-dean








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