Re: Maximum simultaneous outbounds ?
Greetings, Ronald F. Guilmette! >>Postfix is in a different league than software that just runs the >>system into the ground under load, and that requires a babysitter >>to become unstuck. > Thanks for the clarification and the clarity. > You wouldn't happen to have the names of any products that fall > into that other category that you just described would you? > (It really irks me the way that some people demand lots and lots of > IPv4 addresses, which are in short supply, in order to accomplish > things that could be done with lots lots less of that particular > finite and limited resource. But convincing some of these folks > of the error of their ways isn't easy, and I could use all of the > additional ammunition that I can lay hands on.) The main argument you should keep in mind is that connection limit is not "64k", it is "64k from single local //port// to single remote //address//". Which in itself is a hard to even imagine (not to say - reach) situation. There's more realistic limits set in kernel, which will be reached long before that, and which can still be configured to allow for more simultaneous connections, than set by default. Google "c10k problem" for pointers to possible solutions. -- With best regards, Andrey Repin Monday, March 4, 2019 21:49:26 Sorry for my terrible english...
Re: Maximum simultaneous outbounds ?
On Mar 3, 2019, at 16:17, Ronald F. Guilmette wrote: > You wouldn't happen to have the names of any products that fall > into that other category that you just described would you? rsync done this to my system in the past. -- My main job is trying to come up with new and innovative and effective ways to reject even more mail. I'm up to about 97% now.
Re: Maximum simultaneous outbounds ?
Ronald F. Guilmette: > > In message <44c5tp4v0yzj...@spike.porcupine.org>, you wrote: > > >Postfix is in a different league than software that just runs the > >system into the ground under load, and that requires a babysitter > >to become unstuck. > > Thanks for the clarification and the clarity. > > You wouldn't happen to have the names of any products that fall > into that other category that you just described would you? Let's say that Postfix was influenced by good and bad experiences with other software. Wietse
Re: Maximum simultaneous outbounds ?
In message <20190303184645.gl...@straasha.imrryr.org>, Viktor wrote: >I could also point out that TCP stacks can allow the same local >ephemeral port to be used for multiple TCP connections, provided >the 4-tuple (remote ip, remote port, local ip, local port) is unique. >There is no requirement that just the local ports of established >TCP connections be distinct. This answers my original and most fundamental question, and confirms what I believed I already knew about the potential for simultaneous local IPv4 port reuse. So thanks for that. >Well, it seems that you only knew the empirical conclusions. Had you >known how Postfix ensures performance under load, you'd have refuted >the other fellow's false scenario without coming to the list. Well, when arguing (e.g. on a mailing list) with someone who consistantly drops down into the classic retorical "appeal to authority" mode (as in: "I know, you don't, and you are an idiot, so STFU!') it is usually best to get a pronouncement from a a different authority having a different view, if the goal is to refute the false "appeal to authority" being put forward. So I came here. I personally don't know off the top of my head any folks who are more widely considered "authorities" on how mail servers can and should work than you and Wietse. >> I still would like to know if the total number of outbound SMTP connections >> which Postfix may have open, at any one given point in time, may or may not >> exceed 65536. > >This is a silly question. Typical message delivery latency can be >estimated at around 1s. A hypothetical server running at a concurrency >of 64k connections would be pumping out 64k msgs/sec, but the Postfix >queue manager and the disk are very unlikely to go that fast. >Realistically, a single email server may be able to deliver at best >O(1000) msgs/sec. > >At a hypothetical sustained 64k messages per second, a server would >be able to deliver around 5.6 billion messages a day. That's not >a realistic load for a single machine, either inbound or outbound. > >Real servers handle smaller loads with outbound concurrency limits >in the hundreds or a few thousand. With Postfix brief input spikes >that exceed the output rate lead growth in the size of the queue >without unbounded demand for CPU and network. > >There are also caps on concurrent incoming connections, and >sufficiently high input rates will reduce opportunities for new >connections, forcing some or most senders to defer delivery. That's >what horizontal scaling is for, with anycast IPs to spread the load >geographically, and in-datacentre load-balancers to further spread >the load among multiple machines, ... Well, but see, this is precisly what the argument was/is about. As soon as you start talking about load balancers, you are also taking about more than one IP address. It was and is my contention that even great vast gobs of outbound email can be handled on a single IPv4 address, *if* one is doing it "right". And by "right" in this context, I mean having a great big pipe into the machine in question, having the machine itself be something killer, like fer instance a 32-core Ryzen or something, and having the "disk" be something like a 1TB NVME stick, or maybe even... dare I say it?... Optane! Basically, my central thesis in this other conversation that I'm having elsewhere is that current usage norms when it comes to (finite and vanishing) IPv4 addresses are, by and large, exceptionally wasteful and that allocation policy should be adjusted accordingly. My opponents in this debate have used and are using mutiple (mostly lame) arguments for why they need lots and lots of IPv4 addreses. I was able to rather easily shoot down most of those (obviously lame) arguments on my own, but when it came to this question of how many simultaneous outbound mail sessions could dance on the head of a single IPv4 address, I had to ask for some help which I believe I have now, mostly, gotten. (Thank you.) Regards, rfg
Re: Maximum simultaneous outbounds ?
In message <44c5tp4v0yzj...@spike.porcupine.org>, you wrote: >Postfix is in a different league than software that just runs the >system into the ground under load, and that requires a babysitter >to become unstuck. Thanks for the clarification and the clarity. You wouldn't happen to have the names of any products that fall into that other category that you just described would you? (It really irks me the way that some people demand lots and lots of IPv4 addresses, which are in short supply, in order to accomplish things that could be done with lots lots less of that particular finite and limited resource. But convincing some of these folks of the error of their ways isn't easy, and I could use all of the additional ammunition that I can lay hands on.)
Re: Maximum simultaneous outbounds ?
On Sun, Mar 03, 2019 at 01:49:12AM -0800, Ronald F. Guilmette wrote: > >> But this other fellow I've been taking to offered an unexpectedobservation: > >> If a given Postfix installation was attempting to support, say, 1 million > >> unique domain names (correponding to 1 million unique customers) and if > >> just 11,000 of those were to all simultaneously attempt to send -outbound- > >> emails to six (6) different destinations apiece, then... this other fellow > >> asserted... all of the 65536 maximum available IPv4 port numbers would be > >> exhausted, and then havoc would ensue. > > > >This mental model is deeply flawed. > > Thank you for the response Viktor, but could you please be more specific, > just so that I have it on the record? > > Whose mental model is it that you are saying is "deeply flawed"? Mine or > the other guy's? There's only one "mental model" under discussion of what happens when Postfix is delivering email. Namely, that no matter how many messages arrive in quick succession, they'll all be "talking to the network" (using an outbound TCP connection) at the same time. This mental model is deeply flawed. I could also point out that TCP stacks can allow the same local ephemeral port to be used for multiple TCP connections, provided the 4-tuple (remote ip, remote port, local ip, local port) is unique. There is no requirement that just the local ports of established TCP connections be distinct. > Paraphrasing, it sounds to me like you just said that Postfix is designed > to behave well, and in fact does behave well, even under very high loads. I tried to provide a more accurate model of how Postfix delivers email, from which you or anyone else can reach your own conclusions. > But I, for one, already knew that. (And I suspect that most folks who use > Postfix at "big" places knew that already also.) Well, it seems that you only knew the empirical conclusions. Had you known how Postfix ensures performance under load, you'd have refuted the other fellow's false scenario without coming to the list. > I still would like to know if the total number of outbound SMTP connections > which Postfix may have open, at any one given point in time, may or may not > exceed 65536. This is a silly question. Typical message delivery latency can be estimated at around 1s. A hypothetical server running at a concurrency of 64k connections would be pumping out 64k msgs/sec, but the Postfix queue manager and the disk are very unlikely to go that fast. Realistically, a single email server may be able to deliver at best O(1000) msgs/sec. At a hypothetical sustained 64k messages per second, a server would be able to deliver around 5.6 billion messages a day. That's not a realistic load for a single machine, either inbound or outbound. Real servers handle smaller loads with outbound concurrency limits in the hundreds or a few thousand. With Postfix brief input spikes that exceed the output rate lead growth in the size of the queue without unbounded demand for CPU and network. There are also caps on concurrent incoming connections, and sufficiently high input rates will reduce opportunities for new connections, forcing some or most senders to defer delivery. That's what horizontal scaling is for, with anycast IPs to spread the load geographically, and in-datacentre load-balancers to further spread the load among multiple machines, ... -- Viktor.
Re: Maximum simultaneous outbounds ?
Ronald F. Guilmette: > But this other fellow I've been taking to offered an unexpected observation: > If a given Postfix installation was attempting to support, say, 1 million > unique domain names (correponding to 1 million unique customers) and if > just 11,000 of those were to all simultaneously attempt to send -outbound- > emails to six (6) different destinations apiece, then... this other fellow > asserted... all of the 65536 maximum available IPv4 port numbers would be > exhausted, and then havoc would ensue. As shipped, Postfix makes up to 100 parallel outbound connections, 200 if configured as an MX for remote domains. It also has limits on the number and size of in-memory objects, and it stops accepting new mail before the file system is full. Postfix is in a different league than software that just runs the system into the ground under load, and that requires a babysitter to become unstuck. Wietse
Re: Maximum simultaneous outbounds ?
In message <41848ab9-339a-41a8-9a20-b1533eb77...@dukhovni.org>, Viktor Dukhovni wrote: >> On Mar 3, 2019, at 2:56 AM, Ronald F. Guilmette > wrote: >> >> But this other fellow I've been taking to offered an unexpectedobservation: >> If a given Postfix installation was attempting to support, say, 1 million >> unique domain names (correponding to 1 million unique customers) and if >> just 11,000 of those were to all simultaneously attempt to send -outbound- >> emails to six (6) different destinations apiece, then... this other fellow >> asserted... all of the 65536 maximum available IPv4 port numbers would be >> exhausted, and then havoc would ensue. > >This mental model is deeply flawed. Thank you for the response Vicktor, but could you please be more specific, just so that I have it on the record? Whose mental model is it that you are saying is "deeply flawed"? Mine or the other guy's? >Postfix has a queue manager, that >limits the concurrency per destination, and the active queue size. And >a master(8) process that limits the process count per transport. Postfix >also accepts messages at a finite rate, so 66,000 messages will not arrive >instantaneously. Once the active queue is full further accepted messages >will accumulate in the incoming queue on disk, but will not consume network >resources or RAM. Paraphrasing, it sounds to me like you just said that Postfix is designed to behave well, and in fact does behave well, even under very high loads. But I, for one, already knew that. (And I suspect that most folks who use Postfix at "big" places knew that already also.) I still would like to know if the total number of outbound SMTP connections which Postfix may have open, at any one given point in time, may or may not exceed 65536. (I admit that this is really rather entirely a matter of academic curiosity on my part and that it may have little or no practical implications. I just have this running disagreement going about how many angels can dance on the head of... I'm sorry... about how many domain names can, in practice be hosted on a single IPv4 address. I say "millions". Others are telling me that I'm delusional and need to seek immediate treatment. I am not yet favorably inclined to acecpt their judgement on the matter.The key point of disagreement seens to be our differing evaluations about how many simultaneous outbound SMTP a good quality... or best quality... SMTP server could in practice support.) >But the port number exhaustion scenario is not even close. I'm not at all sure how to interpret that. May I assume that your intent was to say that a hosting company could tell all of its 1 million customers to use a single shared mail server for all of their outbound needs, and that even though this might possibly create a unsustainable load, the unsustainability would *not* become evident, in the first instance, as an exhaustion of outbound IPv4 port numbers?
Re: Maximum simultaneous outbounds ?
> On Mar 3, 2019, at 2:56 AM, Ronald F. Guilmette > wrote: > > But this other fellow I've been taking to offered an unexpected observation: > If a given Postfix installation was attempting to support, say, 1 million > unique domain names (correponding to 1 million unique customers) and if > just 11,000 of those were to all simultaneously attempt to send -outbound- > emails to six (6) different destinations apiece, then... this other fellow > asserted... all of the 65536 maximum available IPv4 port numbers would be > exhausted, and then havoc would ensue. This mental model is deeply flawed. Postfix has a queue manager, that limits the concurrency per destination, and the active queue size. And a master(8) process that limits the process count per transport. Postfix also accepts messages at a finite rate, so 66,000 messages will not arrive instantaneously. Once the active queue is full further accepted messages will accumulate in the incoming queue on disk, but will not consume network resources or RAM. It is of course possible to receive inbound messages faster than the steady-state output rate, in which case the number of queued messages will grow quite high. And if this is allowed to continue indefinitely, until the file system almost fills up. But the port number exhaustion scenario is not even close. http://www.postfix.org/OVERVIEW.html#delivering http://www.pos -- Viktor.
