That still irks me. The real problem is not tinygram prevention. It's ACK delays, and that stupid fixed timer. They both went into TCP around the same time, but independently. I did tinygram prevention (the Nagle algorithm) and Berkeley did delayed ACKs, both in the early 1980s. The combination of the two is awful. Unfortunately by the time I found about delayed ACKs, I had changed jobs, was out of networking, and doing a product for Autodesk on non-networked PCs.
Delayed ACKs are a win only in certain circumstances - mostly character echo for Telnet. (When Berkeley installed delayed ACKs, they were doing a lot of Telnet from terminal concentrators in student terminal rooms to host VAX machines doing the work. For that particular situation, it made sense.) The delayed ACK timer is scaled to expected human response time. A delayed ACK is a bet that the other end will reply to what you just sent almost immediately. Except for some RPC protocols, this is unlikely. So the ACK delay mechanism loses the bet, over and over, delaying the ACK, waiting for a packet on which the ACK can be piggybacked, not getting it, and then sending the ACK, delayed. There's nothing in TCP to automatically turn this off. However, Linux (and I think Windows) now have a TCP_QUICKACK socket option. Turn that on unless you have a very unusual application.
Turning on TCP_NODELAY has similar effects, but can make throughput worse for small writes. If you write a loop which sends just a few bytes (worst case, one byte) to a socket with "write()", and the Nagle algorithm is disabled with TCP_NODELAY, each write becomes one IP packet. This increases traffic by a factor of 40, with IP and TCP headers for each payload. Tinygram prevention won't let you send a second packet if you have one in flight, unless you have enough data to fill the maximum sized packet. It accumulates bytes for one round trip time, then sends everything in the queue. That's almost always what you want. If you have TCP_NODELAY set, you need to be much more aware of buffering and flushing issues.
None of this matters for bulk one-way transfers, which is most HTTP today. (I've never looked at the impact of this on the SSL handshake, where it might matter.)
Short version: set TCP_QUICKACK. If you find a case where that makes things worse, let me know.
> it took around 40ms until my application get’s the data. I tested the application on a regular Linux system (Ubuntu) with the same result, so it’s not a RPi limitation. On my OSX MacBook Air however the RPC call needed only 3ms!
One thing that confuses me is -- are ACK delays part of the default TCP implementation on Linux? I originally assumed this was some kind of edge case / unusual behavior.
This is a general problem of leaky abstractions. If you're a top-down thinker, you're going to have a bad time some day and have a hard time figuring it out.
OTOH bottom up thinkers take much longer to become productive in an environment with novel abstractions.
Swings and roundabouts. Top down is probably better in a startup context - it's more conducive to broad and shallow generalists. Bottom up is great when you have a breakdown of abstraction through the stack, or when you need a new solution that's never been done quite the same way before.
Careless piling of layers atop layers is the main reason why everything is slow when computers are crazy fast. Every moderately complex piece of software is so inefficient that it is better not to think about it or else you become paralyzed in horror ;)
Usually something is done to mitigate these inefficiencies only when they become egregious. And that is when even basic knowledge of the inner workings of underlying layers really pays off (see also: mechanical sympathy).
I really enjoy reading Julia's blog. Not only does she have a real, infectious enthusiasm for learning; not only is the blog well written; but I also often learn a lot. Kudos.
Yeah, I was going to post something to the same effect. Her posts (and videos) really drive into not just the how, but the WHY people should care, and her writing is lively and clear. I really love the ambition she has to learn all these things and share it with the world, and to make it inclusive for folks of all experience levels.
John Rauser of pinterest gave a wonderful talk about TCP and the lower bound of Internet latency recently that has a lot in common with what's discussed in the article here. Worth a watch I think if you enjoyed the blog post.
Summary: If you know learn a little, you realize that each packet might be separately acknowledged before the next one is sent. In particular, note this quote: Net::HTTP doesn’t set TCP_NODELAY on the TCP socket it opens, so it waits for acknowledgement of the first packet before sending the second.
By setting TCP_NODELAY, they removed a series of 40ms delays, vastly improving performance of their web app.
You don't need to entirely disable Nagle; just flash TCP_NODELAY on then off immediately after sending a packet for which you will block for a reply. This way you still get the benefit Nagle brings of coalescing small writes, without the downside.
(Alternatively, turn Nagle off entirely and buffer writes manually or using MSG_MORE or TCP_CORK.)
I came across this this week working on the RethinkDB driver for Clojure (https://github.com/apa512/clj-rethinkdb/pull/114). As soon as I saw "40ms" in this story I thought "Nagles Algorithm".
One thing I haven't understood fully is that this only seems to be a problem on Linux, Mac OS X didn't exhibit this behaviour.
Wireshark can show you the delay but it won't tell why it's there. You might assume it's some quirk of your application.. Most people don't consider the kernel/network libraries and drivers.. Those are all black magic
In highschool I carried TCP Illustrated around with me like a bible. I cherished that book. Knowledge of networks would eventually be incredibly useful throughout my career.
This can be generalised. It is also one of my favorite ways of doing developer interviews. Do they have a working/in-depth knowledge of what keeps the inter webs running? So many people have never ventured out of their main competence bubble, and that bubble can be quite small (but focused, I suppose).
For all I know, they believe everything is kept together with the help of magic. I guess I don't trust people who don't have a natural urge to understand at least the most basic things of our foundations.
I used to thing this way before I realized it's just an arbitrary hoop you make people jump through. To you, people understanding TCP might be what you claim is a basic foundation. However, it's just about as arbitrary as asking people to explain 802.11 RTS/CTS or clos switch fabrics, which are both equally as important to delivering day-to-day network traffic. Additionally, they both can come up as things you need to understand when trying to optimize jitter/latency in sensitive local network traffic applications.
Don't judge people based on which components of networks they happened to take an interest in and dive into.
Sounds exactly like what TCP_NODELAY does on linux.
Note the final sentence from tcp(7):
TCP_NODELAY
If set, disable the Nagle algorithm. This means that segments are always sent as soon as possible, even if there is
only a small amount of data. When not set, data is buffered until there is a sufficient amount to send out, thereby
avoiding the frequent sending of small packets, which results in poor utilization of the network. This option is
overridden by TCP_CORK; however, setting this option forces an explicit flush of pending output, even if TCP_CORK is
currently set.
Animats|10 years ago
Delayed ACKs are a win only in certain circumstances - mostly character echo for Telnet. (When Berkeley installed delayed ACKs, they were doing a lot of Telnet from terminal concentrators in student terminal rooms to host VAX machines doing the work. For that particular situation, it made sense.) The delayed ACK timer is scaled to expected human response time. A delayed ACK is a bet that the other end will reply to what you just sent almost immediately. Except for some RPC protocols, this is unlikely. So the ACK delay mechanism loses the bet, over and over, delaying the ACK, waiting for a packet on which the ACK can be piggybacked, not getting it, and then sending the ACK, delayed. There's nothing in TCP to automatically turn this off. However, Linux (and I think Windows) now have a TCP_QUICKACK socket option. Turn that on unless you have a very unusual application.
Turning on TCP_NODELAY has similar effects, but can make throughput worse for small writes. If you write a loop which sends just a few bytes (worst case, one byte) to a socket with "write()", and the Nagle algorithm is disabled with TCP_NODELAY, each write becomes one IP packet. This increases traffic by a factor of 40, with IP and TCP headers for each payload. Tinygram prevention won't let you send a second packet if you have one in flight, unless you have enough data to fill the maximum sized packet. It accumulates bytes for one round trip time, then sends everything in the queue. That's almost always what you want. If you have TCP_NODELAY set, you need to be much more aware of buffering and flushing issues.
None of this matters for bulk one-way transfers, which is most HTTP today. (I've never looked at the impact of this on the SSL handshake, where it might matter.)
Short version: set TCP_QUICKACK. If you find a case where that makes things worse, let me know.
John Nagle
tptacek|10 years ago
I wish you hadn't signed your comment, so we could have had the "I am John Nagle" moment when someone inevitably tried to pedantically correct you. :)
masklinn|10 years ago
Apparently Greg Minshall proposed tinygram prevention alternations 15 years ago to fix the problematic interaction: https://tools.ietf.org/html/draft-minshall-nagle-01
OSX seems to have implemented this in 2007 and be less/not sensitive to the issue e.g. http://neophob.com/2013/09/rpc-calls-and-mysterious-40ms-del... notes that there was no delay on OSX
> it took around 40ms until my application get’s the data. I tested the application on a regular Linux system (Ubuntu) with the same result, so it’s not a RPi limitation. On my OSX MacBook Air however the RPC call needed only 3ms!
jvns|10 years ago
Dylan16807|10 years ago
barrkel|10 years ago
OTOH bottom up thinkers take much longer to become productive in an environment with novel abstractions.
Swings and roundabouts. Top down is probably better in a startup context - it's more conducive to broad and shallow generalists. Bottom up is great when you have a breakdown of abstraction through the stack, or when you need a new solution that's never been done quite the same way before.
dunkelheit|10 years ago
Usually something is done to mitigate these inefficiencies only when they become egregious. And that is when even basic knowledge of the inner workings of underlying layers really pays off (see also: mechanical sympathy).
the8472|10 years ago
jfb|10 years ago
bufordsharkley|10 years ago
p00b|10 years ago
https://www.youtube.com/watch?v=C8orjQLacTo
PeterWhittaker|10 years ago
By setting TCP_NODELAY, they removed a series of 40ms delays, vastly improving performance of their web app.
colanderman|10 years ago
(Alternatively, turn Nagle off entirely and buffer writes manually or using MSG_MORE or TCP_CORK.)
dantiberian|10 years ago
One thing I haven't understood fully is that this only seems to be a problem on Linux, Mac OS X didn't exhibit this behaviour.
tirumaraiselvan|10 years ago
bboreham|10 years ago
neduma|10 years ago
spydum|10 years ago
jvns|10 years ago
rjurney|10 years ago
mwfj|10 years ago
For all I know, they believe everything is kept together with the help of magic. I guess I don't trust people who don't have a natural urge to understand at least the most basic things of our foundations.
hueving|10 years ago
Don't judge people based on which components of networks they happened to take an interest in and dive into.
Ono-Sendai|10 years ago
Animats|10 years ago
daurnimator|10 years ago
Note the final sentence from tcp(7):