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4.12. What about VERP?

See also <So what is this VERP stuff> and <What MTA features are required for use with Mailman 2.1.5?>.

Here's one example of how this works in practice, summarizing various messages in the thread at <

Enabling VERP can be a serious performance hit for large lists. One example list with over 150,000 recipients saw a factor of 25-75 reduction in throughput, when VERP was enabled:

   May 27 16:43:46 2004 (440) <20040527135407.74A5F368143 at> smtp
        for 151942 recips, completed in 1231.438 seconds
   Jun 11 19:05:45 2004 (440) <20040611163245.ED9CB3680AE at> smtp
        for 152333 recips, completed in 649.634 seconds
   Jun 30 15:39:26 2004 (435) <20040630132741.F1A0836811C at> smtp
        for 152717 recips, completed in 428.891 seconds
   Jul 13 02:05:22 2004 (435) <20040712150834.DCA153680BD at> smtp
        for 152991 recips, completed in 31782.241 seconds


   428.891/152717 = 0.0028084 seconds per recipient (average)
   1231.438/151942 = 0.0081046 seconds per recipient (average)
   31782.241/152991 = 0.2077392 seconds per recipient (average)

This may not sound like much, but keep in mind that 31782.241 seconds is 529.704 minutes, or 8.828 hours, and during that time the mailing-list system was pretty much stuck (the other mail, that did not need to pass through Mailman, was OK).

If you can modify your Mailman configuration to let the MTA do the VERPing, you won't get the personalization benefits, but you will get the improved bounce management benefits. Both Exim and postfix are capable of doing this. For postfix, see <>, although this will currently require that you modify your Mailman source code to use the "XVERP" option.

Of course, in this example the machine may not have been tuned for maximum performance, so your results might be better, if you pay more attention to the tuning of your systems. Of course, your results could also be worse.

So much for one example of the practice.

Now, here's the theory, from a message by Chuq von Rospach at <>:

  1000 subscribers -- no digest subscribers to simplify this. Assume
  just individual messages.

  The message size is 10K, including header.

  The bandwidth needed to generate a connection to send a message is 1K
  (which is pretty close)

  The bandwidth needed to add an address to an existing message is about
  1/10 of a K (also pretty close).

  The practical limit to the number of messages you can piggyback is
  100, since this is specified in RFC2821 as the smallest number a site
  is REQUIRED to take. In practice, due to non-conformant sites, you
  have to be careful setting it beyond 50 these days, because sites set
  this number down because they think it slows down the spammers (I'm
  yet to be convinced it makes a damn bit a difference, especially since
  MTAs like postifx recognize the 452 and auto-adjust now. This is
  another place where sendmail seems behind the technology curve, FWIW)

  How much bandwidth is used depends on these factors:

  what your piggyback value is (in mailman, it's SMTP_MAX_RCPTS)

  how many domains have &gt; 1 subscriber.

  Here's how plaidworks breaks down:

    3101 subscribers across 1287 domains. that's an average of 2.3
    subscribers per domain, but the numbers skew wildly, so averages
    are meaningless.

  Here's how my site breaks down:

    # of subscribers                    # of domains/# of users
    ---------------------                       -----------------
    1                                           263/263
    2                                           142/284
    3                                           40/120
    4                                           19/76
    5                                           16/80
    6                                           10/60
    7                                           7/49
    8                                           3/24
    9                                           6/54
    10                                          2/20
    11                                          2/22
    12                                          2/24
    13                                          1/
    14                                          1/
    16                                          1/
    17                                          1/ (
    22                                          1/(
    29                                          1 (
    30                                          1 (
    35                                          1 (
    43                                          1 (
    53                                          1 (
    150                                         1 (
    173                                         1 (
    228                                         1 (
    441                                         1 (

    if you're scoring at home, 37% of subscribers come from that last
    4 domains: 5% for home and yahoo, 7% for hotmail, and 14% for aol.
    those are your 500 pound gorillas (AOL is 800 pounds), and piss
    them off at your own risk.

  At the other end, 8% of your users are the only subscriber from
  a domain. 16% are 1 or 2 per domain. 26% are on sites with 5 or
  fewer subscribers.

  Time for some numbers.

  Back to the 1000 member list for simplicity. The subscriber list
  breaks down to:

    85  -       1/85
    45  -       2/90
    12  -       3/36
    6   -       4/24
    48  -       1
    55  -       1
    73  -       1
    142 -       1

  That's 553, or 55% of the subscribers, wedged tightly on both ends
  of the curve. We can extrapolate what they'll do to bandwidth from
  the end cases if we need to.

    Extreme case: SMTP_MAX_RCPTS = 1.

    1000 subscribers * (10K message size + 1K overhead) = 11,000K bytes

    Extreme case: SMTP_MAX_RCPTS = 100

  These get sent  down the line this way:

    85 * 11K
    45 * (1 * 11K + 1 * .1K)
    12 * (1 * 11K + 2 * .1K
    6 * (1 * 11K + 3 * .1K)
    1 * 11K + 47 * .1K
    1 * 11K + 54 * .1K
    1 * 11K + 72 * .1K
    2 * 11K + 140 * .1K

  Do you see how I got these numbers? In the case of the 12 domains
  with three subscribers, you have to make an 11K connection for the
  first message, and piggy back on the other two addresses at .01K each.
  You don't really see huge savings until the big domains, and you'll
  see AOL goes over the 100 address limit so gets split into two
  different messages.

  For this 55%, the SMTP=1 is 6050K. For 100, it's 1711K bytes. That's
  28% of the first number, so we're cutting 72% of the bandwidth by
  chunking at 100. The tradeoff is performance, though -- it takes a
  lot longer to deliver those AOL addresses, because if you split it
  into two batches, you can't parallelize the delivery. Package up 100
  AOL addresses in one batch, none of them get delivered until all 100
  addresses are sent to AOL and accepted. It's much faster to send them
  as ten batches of ten in parallel -- but that's the trade off here.
  Cut network bandwidth but slow delivery to the larger domains.

  Okay, let's look at a case in the middle. SMTP_MAX = 5. The ones with
  less than 5 don't change, but the big domains do

    85 * 11K
    45 * (1 * 11K + 1 * .1K)
    12 * (1 * 11K + 2 * .1K
    6 * (1 * 11K + 3 * .1K)
    1 * (10 * 11K + 38 * .1k)
    1 * (11 * 11K + 44 * .1K)
    1 * (15 * 11K + 58 * .1K)
    1* (29 * 11K + 113 * .1K)

  that works out to (trust me) about 2378K, or about a 60% reduction.

  Let's try SMTP_MAX = 2.

    85 * 11K
    45 * (1 * 11K + 1 * .1K)
    12 * (2 * 11K + 1 * .1K
    6 * (2 * 11K + 2 * .1K)
    1 * (10 * 11K + 38 * .1k)
    1 * (11 * 11K + 44 * .1K)
    1 * (15 * 11K + 58 * .1K)
    1* (29 * 11K + 113 * .1K)

  that works out to 2575K, or about a 57% cut.

  By a rough look at those domains in the middle, I'd say these numbers
  are good +-10%.

  What's this mean? Here's the executive summary:

  The network penalty between SMTP_MAX = 1 (effectively VERP) and any
  kind of batching (SMTP &gt; 1) is roughly 50%. To get VERP or customized
  footers or customized anything, you double your network bandwidth.

  There is very little advantage to setting SMTP_MAX &gt; 5, UNLESS your
  subscriber base is heavily stratified onto very few sites. If you
  have really large groups of subscribers on AOL or Hotmail, it can help
  cut network bandwidth, but at best, it seems to be about a 10%

  If you plot the numbers I did on a curve, you can see just how
  little advantage you get by increasing the number. You get almost
  all of the advantage by going to 2, and the line past 5 is very

  Interesting -- I honestly didn't expect to see THIS big a difference
  -- I was expecting more like 25-30% increase in bandwidth for a
  VERP-type delivery.

  My thoughts on what this means to future directions:

    Customized messages (VERPing, or encoded unsub URLs, or all of
    that...) should definitely be an option in Mailman 2.1.

  I would set Mailman's 2.1 default to have this turned ON, giving us
  the customized unsub links and etc, but to document this for users so
  they know to turn it off on slow networks.

  If users turn it off, I recommend that SMTP_MAX be set by default to
  5, and that we document that it makes little sense to change it
  unless a site is horribly network limited, because even setting to
  the max only gains them another 10% (and if they're THAT network
  limited, they're seriously asking for trouble anyway), and only if
  their subscriber base fits a profile that lends itself to the
  compression. Setting it large also leaves them open to spamblocking
  by systems that don't necessarily follow the standards or act right,

  We should ALSO note here that some MTAs (postfix, for instance)
  might override SMTP_MAX anyway -- you could set it to 100, but
  postfix might be configured smaller, so they have to be aware of
  those potential interactions. you then get into the issues of
  tuning all this, with few delivery threads with lots of addresses
  vs many threads in parallel.. and all that fun -- I guess I'm
  trying to say that you can't tune mailman in isolation from the
  MTA (and down that road lies a huge rathole of attempting to
  document this stuff...)

  But from these numbers, any 2.0.x version of mailman should set
  SMTP_MAX to between 2 and 5, unless they're horribly network
  limited. it makes no sense to be larger than 5, and it makes no
  sense to be 1 unless you've done some kind of VERPing patch.

  for 2.1, we want to implement these customizations and default them
  on, but with a 50% network hit, we definitely want to make it clear
  what's going on and make it possible for them to turn it off and
  return to a generic URL and non-customized e-mail.

  Barry's mileage may vary on his preferences for default, of course,
  and it's his show. I think the advantages of the customized URL/email
  capability is a huge one and most sites will benefit from it -- but
  the network hit might kill some sites, so we have to give them an easy
  ability to turn the feature off.

  What do y'all think? I've included mailman-developers on this reply,
  since while this started on mm-users, it really ought to be discussed
  on the developers list...

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MailmanWiki: DOC/4.12 What about VERP? (last edited 2015-01-31 02:36:58 by msapiro)