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Re: New document on "Simple hit-metering for HTTP"

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Wed, 07 Aug 96 16:00:22 MDT
Message-Id: <9608072300.AA01901@acetes.pa.dec.com>
To: Koen Holtman <koen@win.tue.nl>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
    But to allocate max-uses values to proxies an efficient way, an
    origin server seems to have to keeping per-proxy database of
    `max-use-qouta-use-speed' (last two paragraphs of Section 2), which
    adds some overhead to every request.

Only if it really wants to be cautious about bounding the counting
error.  I would imagine that the simplest servers would have a global
setting for this value (e.g., always send "max-uses=10").  A somewhat
more sophisticated server might keep a moderate-sized cache of
(proxy-address, max-use-setting) entries.  At about 8 bytes per
entry (4 bytes IP address, 2 bytes max-use-setting, perhaps 2 bytes for
an LRU counter), you could spend about a dime on RAM and keep 1000 such
entries.

    Reading these paragraphs, the
    goal of the max-uses allocation heuristics seem to be to ensure
    that all counts are reported `soon enough'.

"Soon" in the sense of "bounding the error in the final count", not
in the sense of "within 3 hours".

    It seems that a max-time-to-wait-before-reporting-hits mechanism,
    can achieve the same goal without the same computational overhead
    in origin servers.  This mechanism would also eliminate the need
    for implementing difficult max-use distribution heuristics in proxy
    caches: a cache could simply subtract the age of the response from
    the max-time value.

    Even better, we *already have* a
    max-time-to-wait-before-reporting-hits mechanism in the form of
    cache-control: max-age.

    I conclude that the max-use mechanism is unnecessary and propose
    that it is removed, and that a section about using cache-control:
    max-age is added.

I think this would be a mistake, since it doesn't distinguish between
very busy caches and very lightly-used ones.  If you send a small
max-age value, then the lightly-used caches might report far more
often than necessary.  If you send a large max-age value, then the
heavily-used caches might report far too infrequently (in terms
of number of uses between reports).

-Jeff
Received on Wednesday, 7 August 1996 16:11:58 EDT

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