[EMAIL PROTECTED] wrote:
>On Mon, 8 Nov 1999, Dave Sill wrote:
>
>> Say you send a message to a list of 10,000 addresses using
>> sendmail. What's the first thing it does? It looks up the MX for each
>> recipient so it can sort by MX and minimize the number of connections.
>
>I doubt very much that that's what sendmail does, by the simple fact of
>what would happen if the best MX is unreachable. The resulting logic
>would be too convoluted even for sendmail, because then it would have to
>reshuffle every message, since you certainly can't assume that all domains
>pointing to the same best MX will also have the same second-best MX listed
>as well.
>
>I think you're really referring to the behavior of older sendmails which
>have a pathological need to issue a CNAME query for every address in the
>headers in order to rewrite it in those few cases where there's a CNAME
>record for the domain. I don't think sendmail does that anymore.
I don't really know what sendmail does these days, nor do I care. When
I last used sendmail, I know it spent a great deal of time thinking
before delivering the first message to a moderate list of recipients.
>> qmail, on the other hand, fires off concurrencyremote qmail-remotes
>> and starts delivery immediately.
>
>... But not after issueing the same DNS query to find the MX for each
>recipient. qmail-remote does not pull an IP address out of thin air.
Each qmail-remote does one DNS lookup, so at most concurrencyremote
DNS lookups are concurrent, and qmail doesn't have to wait for all of
them before sending any messages.
>Additionally, I'm not sure but it's possible that sendmail will not query
>for the same domain the second time, thus 10,000 messages going to 5,000
>different domains will result in only 5,000 DNS queries. Meanwhile, each
>instance of qmail-remote should diligently issue a DNS query - for a
>grand sum of 10,000 queries overall.
>
>So what you have here is 10,000 guaranteed queries from Qmail, and up to
>10,000 DNS queries for sendmail, maybe less.
Maybe more. Back in the olden days, when I last used sendmail, it did
multiple DNS lookups per delivery.
>> I think Postfix just sorts by FQDN, so it doesn't have to do 10,000
>> DNS lookups before it starts delivering. But by doing that, it
>> potentially misses a lot of combining for different FQDN's with the
>> same MX.
>
>I happen to think that this is a reasonable compromise.
Agreed.
But, as the master says: "Profile, don't speculate". Take a look at:
http://www.kyoto.wide.ad.jp/mta/eval1/perf1b.gif
Which shows that in a test where sendmail, qmail, postfix, and other
UNIX MTA's were given the same task, qmail was fastest. And:
http://www.kyoto.wide.ad.jp/mta/eval1/perf1b+.gif
Which shows that qmail started delivering messages fastest.
There are numerous other charts showing qmail's measured superiority
on:
http://www.kyoto.wide.ad.jp/mta/eval1/
-Dave
