Re: ICP & HEAD requests

From: Chris Wedgwood <>
Date: Mon, 7 Sep 1998 11:01:37 +1200

On Sun, Sep 06, 1998 at 09:50:14PM +1000, Anthony Baxter wrote:

> If 'ICP-with-a-head' has to hit disk, to get the headers of the
> object, for each ICP request, suddenly ICP becomes a serious
> serious load.

OK, this makes sense.

> That's why we killed UDP_HIT_OBJ ages and ages ago, and I think
> it's now totally gone from squid - you need to get the ICP out
> fast.

Checking the access logs, HEADs aren't all that common, and typically
access patterns are HEAD then GET, so in theory I guess the bulk of
the data will be transfered from a sibling anyhow. I did notice that
almost all HEAD requests are for the same very small number of URLs.
It looks like things such as the Yahoo Pager which does auto-update
uses HEAD to check for a newer version.

One cheap optimization I can think of, is that when a HEAD is issued,
the URL is cached for a very short amount of time, 15s perhaps, and a
maximum of 100 or so, and if the URL is then fetched within this
period of time from a sibling that is marked proxy-only, it will
actually store the object locally (ignoring the proxy-only status of
the sibling).

I'm not sure how much this would gain though, a second or so faster
when issuing a HEAD, a small number of ICP requests, for the sake of
0.005% less data, if that?

Received on Tue Jul 29 2003 - 13:15:53 MDT

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