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What happens if you write a TCP stack in Python?

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Re: What happens if you write a TCP stack in Python?

#91

Post by srean »

Too late to join the story, but I am really curious if datacenter nodes intended for heavy mapreduce use implement this layer in user space.

The bottleneck for such processes is typically network I/O and I can imagine that taking control of the network in the user space might offer some modest to significant wins. For Hadoop in particular network packets needs to traverse quite a few layers before it is accessible to the application.

Has anyone done this sort of a thing for mapreduce. Pointers to any writeup would be awesome.

In fact TCP itself might be an overkill for mapreduce. The reduce functions used are typically associative and commutative. So as long as the keys and values are contained entirely within a datapacket, proper sequencing is not even needed. Any sequence would suffice.

Re: What happens if you write a TCP stack in Python?

#92

Post by meowface »

srean wrote: post #91

Too late to join the story, but I am really curious if datacenter nodes intended for heavy mapreduce use implement this layer in user space. The bottleneck for such processes is typically network I/O and I can imagine that taking control of the network in the user space might offer some modest to significant wins. For Hadoop in particular network packets needs to traverse quite a few layers before it is accessible to…

I believe Google has experimented with this sort of thing, as have a few other companies that sell a supposedly more efficient/faster userspace TCP stack as an enterprise product.

Re: What happens if you write a TCP stack in Python?

#93

Post by [deleted] »

cbhl wrote: post #71
tptacek wrote: post #23

The idea that Python is so slow that it's confusing TCP sounds wrong to me. I think it's more likely that your packet capture scheme is slow. It looks like you're using scapy, which I assume is in turn using libpcap... which may be buffering (in high-performance network monitoring, the packet capture interface goes out of its way to buffer). Which is something you can turn off. About 13 years ago, I wrote my own prog…

The Google homepage is only about 20000 bytes... if we assume an maximum segment size of ~1400 bytes, then 14 or 15 packets is about right. I wouldn't be surprised if Google is sending the packets all at once and ignoring the ACKs altogether. Heck, there's even a 2010 paper from Google on the subject of sending a bunch of packets at the beginning of the connection: An Argument for Increasing TCP's Initial Congestion…

[deleted]

Re: What happens if you write a TCP stack in Python?

#94

Post by dragonwriter »

blutoot wrote: post #84
blutoot wrote: post #57

Earlier quoted context omitted.

I believe the author is a "she". It says at the banner of the blog.

I think this downvoting thing is out of control. What was even remotely offensive about this correction?

"offensive" isn't the only reason for downvoting. "Not a substantive addition to the conversation" is one (of many) other bases. If you are going to complain about downvoting [1] why do so based on the unwarranted assumption that the downmod must be for offensiveness?

[1] And you shouldn't, see the Guidelines [2] under "In Comments"

[2] https://news.ycombinator.com/newsguidelines.html

Re: What happens if you write a TCP stack in Python?

#95

Post by wmf »

srean wrote: post #91

Too late to join the story, but I am really curious if datacenter nodes intended for heavy mapreduce use implement this layer in user space. The bottleneck for such processes is typically network I/O and I can imagine that taking control of the network in the user space might offer some modest to significant wins. For Hadoop in particular network packets needs to traverse quite a few layers before it is accessible to…

There has been some work about using zero-copy RDMA and RoCE (which bypass the kernel as a side effect) for analytics. Hadoop in particular is so slow that the kernel is unlikely to be a bottleneck, but more optimized runtimes like Spark might benefit.

Re: What happens if you write a TCP stack in Python?

#96

Post by srean »

wmf wrote: post #95
srean wrote: post #91

Too late to join the story, but I am really curious if datacenter nodes intended for heavy mapreduce use implement this layer in user space. The bottleneck for such processes is typically network I/O and I can imagine that taking control of the network in the user space might offer some modest to significant wins. For Hadoop in particular network packets needs to traverse quite a few layers before it is accessible to…

There has been some work about using zero-copy RDMA and RoCE (which bypass the kernel as a side effect) for analytics. Hadoop in particular is so slow that the kernel is unlikely to be a bottleneck, but more optimized runtimes like Spark might benefit.

I dont understand why is Hadoop so freaking slow. I am no fan of Java (to put it mildly) but Java does fairly well to keep in the 70~80% of a C++ code at the cost of 4 to 5 times more memory. My experience is that Hadoop is 4 to 6 times slower.

Is it because of bad choice of internal algorithms, bad choice of internal data structures ? Bad I/O design ? Given the popularity it enjoys, and given its age, its a little frightening how much worse Hadoop is in comparison to some proprietary implementations. My hunch is that Hadoop's slowdown is in the shuffle phase, which is where faster network data transfer can help.

I like abstractions that spark exposes but it still needs a lot of engineering to catchup. I have anecdotes where Spark is slower than Hadoop by quite a bit.

All my experience is with Hadoop 1.x. Is Hadoop 2.x much better ?

Re: What happens if you write a TCP stack in Python?

#98

Post by vacri »

dragonwriter wrote: post #94
blutoot wrote: post #84

Earlier quoted context omitted.

I think this downvoting thing is out of control. What was even remotely offensive about this correction?

"offensive" isn't the only reason for downvoting. "Not a substantive addition to the conversation" is one (of many) other bases. If you are going to complain about downvoting [1] why do so based on the unwarranted assumption that the downmod must be for offensiveness? [1] And you shouldn't, see the Guidelines [2] under "In Comments" [2] https://news.ycombinator.com/newsguidelines.html

In my opinion, if HN doesn't want puzzled people to ask why they've been downmodded, then instead of putting a line in the Guidelines saying "don't do this", they should separate the downmod into "I disagree" and "flag this as a bad comment", with the latter item the one that reduces the text weight. It's clearly broken - people have been doing this for as long as I've been here, and it's not going to change (sadly, 'it' means both the behaviour, and HN's braindead downmod mechanism).

Re: What happens if you write a TCP stack in Python?

#99

Post by qewrffewqwfqew »

digitalsushi wrote: post #79
tptacek wrote: post #23

The idea that Python is so slow that it's confusing TCP sounds wrong to me. I think it's more likely that your packet capture scheme is slow. It looks like you're using scapy, which I assume is in turn using libpcap... which may be buffering (in high-performance network monitoring, the packet capture interface goes out of its way to buffer). Which is something you can turn off. About 13 years ago, I wrote my own prog…

Our tcp stack is written in tcl. The window size is set to 1. It works fine.

As a heavy user of Tcl, I'm interested to know more. Is this related to hping3 or pktsrc or NS? Or are you F5?

Re: What happens if you write a TCP stack in Python?

#100

Post by voltagex_ »

jnbiche wrote: post #51

This is a fun write-up. If you enjoy this kind of playing around with networking in a dynamic language, and don't want to have to worry about ARP spoofing to do these kinds of experiments, you may want to take a look a Snabb Switch. It provides userland networking in Lua, connecting to the NIC directly (only a handful of popular NICs currently supported) [0]. I've not used it yet, but I've read over the documentation…

I can't find the list of supported NICs in the documentation, but I found https://github.com/SnabbCo/snabbswitch/blob/master/src/lib/h... which suggests that only a very small subset of Intel NICs are supported. One of those may be emulated by VirtualBox, though.
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