The curious case of slow downloads(blog.cloudflare.com)
blog.cloudflare.com
The curious case of slow downloads
https://blog.cloudflare.com/the-curious-case-of-slow-downloads/
9 comments
> For example, Microsoft may be changing their image, but their core software is closed source. <cloud provider> may be great, but can you pull off something like this when you've got an issue? The importance of being able to debug and patch your mission critical systems is hard to overstate.
Not OSS != no source access. A cloud provider on top of the MS stack would most likely have Shared Source Initiative licenses.
Not OSS != no source access. A cloud provider on top of the MS stack would most likely have Shared Source Initiative licenses.
Cloudflare was allowed to patch their nginx with the correct solution. Would the Shared Source Initiative allow you to do that, ie patch and run the modified version ?
I'm guessing no, since the licenses on the SSI site include:
Licensees may use the source code as a reference or view within a debugger,
but may not modify the code or use it to create derivative works.> Not OSS != no source access.
Furthermore, no source access != no way to fix problems.
People have been fixing things with no source for ages, EULAs be damned. If the fix is literally flipping a single bit, the decision is easy. Just do it. The vendor doesn't need to know nor care what we did to fix it, but often giving a detailed description to them of the problem --- one that comes out of the process of fixing it --- will help them fix it quickly and distribute to their other customers too. I've had this experience a few times.
Obviously, "the byte at 0x73F441 of somefile.dll should be 31, not 32" is not how you should communicate such things to the vendor, but they do appreciate your effort in debugging the issue instead of immediately blaming it on a fault in their software. ;-)
(And relatedly, no source access != no way to find problems either, as the security community shows quite plainly...)
Furthermore, no source access != no way to fix problems.
People have been fixing things with no source for ages, EULAs be damned. If the fix is literally flipping a single bit, the decision is easy. Just do it. The vendor doesn't need to know nor care what we did to fix it, but often giving a detailed description to them of the problem --- one that comes out of the process of fixing it --- will help them fix it quickly and distribute to their other customers too. I've had this experience a few times.
Obviously, "the byte at 0x73F441 of somefile.dll should be 31, not 32" is not how you should communicate such things to the vendor, but they do appreciate your effort in debugging the issue instead of immediately blaming it on a fault in their software. ;-)
(And relatedly, no source access != no way to find problems either, as the security community shows quite plainly...)
What's that cost? How quickly can it be done? If I was a developer would I have to ask my manager to look into the details of our support plan? What about the ability to patch the source?
`git clone` is an incredible power. Companies should reward those who are generous with their code.
`git clone` is an incredible power. Companies should reward those who are generous with their code.
> What's that cost? How quickly can it be done?
Careful with those goalposts, you're tearing up the pavement.
> Companies should reward those who are generous with their code.
So companies "should reward those who are generous with their code" but not compensate them for it?
Careful with those goalposts, you're tearing up the pavement.
> Companies should reward those who are generous with their code.
So companies "should reward those who are generous with their code" but not compensate them for it?
> So companies "should reward those who are generous with their code" but not compensate them for it?
No, they absolutely should compensate them for it. Buy OSS.
No, they absolutely should compensate them for it. Buy OSS.
"So you're asking me to pay for something we can get for free" said every Finance Officer ever...
Indeed the financial support of open source by many users is atrocious. Beyond that you're proselytizing.
This issue didn't get solved particularly quickly, either. Nor was "source to the whole stack" required -- kernel source made for a illustrative blog post but was not really part of the process.
Do you have source files for the CPU? The motherboard? Your DRAM controller? At some point you draw the line and make shit work. Tools plus docs plus code is great, but you can make do with just one of 'em in a pinch.
This issue didn't get solved particularly quickly, either. Nor was "source to the whole stack" required -- kernel source made for a illustrative blog post but was not really part of the process.
Do you have source files for the CPU? The motherboard? Your DRAM controller? At some point you draw the line and make shit work. Tools plus docs plus code is great, but you can make do with just one of 'em in a pinch.
There's a different model in this case. You pay for support, and then the original developers go away and fix it for you.
When this works well, it's great. You've got the experts on the problem. Not all support is good though, obviously, and if you have a mission critical problem that you need to fix overnight, nothing beats the 'self-support' that you can do with open source software.
When this works well, it's great. You've got the experts on the problem. Not all support is good though, obviously, and if you have a mission critical problem that you need to fix overnight, nothing beats the 'self-support' that you can do with open source software.
Totally! Pay for support works when it's not urgent. But when your biz is hurting, it's hard to translate that urgency to another company that has _entirely_ different priorities than you do.
Reminds me of the heroku shaming posts that (then) rap genius had to do regarding dyanamo scaling and performance - http://genius.com/James-somers-herokus-ugly-secret-annotated They had no other way forward, even though they were paying the "experts"
This is why companies should pay open source contributors more frequently :-)
Reminds me of the heroku shaming posts that (then) rap genius had to do regarding dyanamo scaling and performance - http://genius.com/James-somers-herokus-ugly-secret-annotated They had no other way forward, even though they were paying the "experts"
This is why companies should pay open source contributors more frequently :-)
They just rediscovered silly window syndrome, but at the application level rather than the TCP level.
In early TCP implementations, if you read from a TCP connection one byte at a time, and the sender was writing faster than the receiver, the sender TCP would get an ACK opening one byte of window. The sender TCP would then transmit a packet with only one byte. This often came up if a TCP connection was driving a slow device. The fix was to not send a window update until there was at least one full packet worth of window available.
This is the same problem, but at the OS buffering level. Linux has a similar solution. The problem is 1) servers today have to protect themselves against readers that are "too slow", because that can be used as a denial of service attack, and 2) the approach to doing this was not well designed.
This is a tough one for Cloudflare, because they're in the DDOS-prevention business. They have to decide whether a slow reader is an attacker or just someone on a slow connection. Still, insisting that a reader consume at least 5MB/minute is a bit much. Some people are still on dialup.
In early TCP implementations, if you read from a TCP connection one byte at a time, and the sender was writing faster than the receiver, the sender TCP would get an ACK opening one byte of window. The sender TCP would then transmit a packet with only one byte. This often came up if a TCP connection was driving a slow device. The fix was to not send a window update until there was at least one full packet worth of window available.
This is the same problem, but at the OS buffering level. Linux has a similar solution. The problem is 1) servers today have to protect themselves against readers that are "too slow", because that can be used as a denial of service attack, and 2) the approach to doing this was not well designed.
This is a tough one for Cloudflare, because they're in the DDOS-prevention business. They have to decide whether a slow reader is an attacker or just someone on a slow connection. Still, insisting that a reader consume at least 5MB/minute is a bit much. Some people are still on dialup.
Great that they solved it! My company (degoo.com) had to use CloudFront instead of CloudFlare for our binaries because of this bug. Our conversion rate went down about 10% whenever we used CloudFlare instead. Perhaps time to switch back.
Original title doesn't mention "NGINX".
Context in a title is useful.
> Not an NGINX problem either
There is a header that says that, but it actually was an NGINX problem.
Not really. nginx's implemented behaviour seems more or less what anyone writing socket code would do: write to buffer, then wait with timeout to write more, if timeout is hit consider the write failed.
The problem here is that (TIL) on linux the "writable socket" state behaves completely differently than the "readable socket" one, and a socket can be effectively writeable even though poll(2) reports that it isn't.
The problem here is that (TIL) on linux the "writable socket" state behaves completely differently than the "readable socket" one, and a socket can be effectively writeable even though poll(2) reports that it isn't.
The TCP layer is already perfectly capable of detecting if a connection has failed. NGINX wanted to do something extra, and did it incorrectly.
nginx is also resetting the connection instead of closing it. That's also a bug (separately). The use of a TCP reset here violates RFC and Postel's robustness maxim. Be liberal on what you accept and be conservative in what you send.
> NGINX wanted to do something extra, and did it incorrectly.
Having a timeout on a poll (or select) is not "something extra". it's something entirely normal and necessary for any non-trivial software, especially public-facing ones.
Having a timeout on a poll (or select) is not "something extra". it's something entirely normal and necessary for any non-trivial software, especially public-facing ones.
What? They wanted a timeout on how long it takes X bytes to send on an active TCP connection. That has nothing to do with poll/select timeouts.
> What? They wanted a timeout on how long it takes X bytes to send on an active TCP connection. That has nothing to do with poll/select timeouts.
nginx called send(file), then polled the socket for write with the timeout, closing the connection if the timeout was hit. It has everything to do with poll/select timeouts, and especially with Linux letting poll hit timeouts when waiting on effectively writable sockets.
nginx called send(file), then polled the socket for write with the timeout, closing the connection if the timeout was hit. It has everything to do with poll/select timeouts, and especially with Linux letting poll hit timeouts when waiting on effectively writable sockets.
> writable sockets
The goal of NGINX here is not to measure whether a socket can consume any data whatsoever. A connection that consumes 1 byte every 15 seconds is still considered stalled, and should be killed. Using the time between sendfile invocations is not the goal, it's a means toward implementing a minimum rate, and they implemented a minimum rate the wrong way. It's an X Y problem and that's not the kernel's fault.
The goal of NGINX here is not to measure whether a socket can consume any data whatsoever. A connection that consumes 1 byte every 15 seconds is still considered stalled, and should be killed. Using the time between sendfile invocations is not the goal, it's a means toward implementing a minimum rate, and they implemented a minimum rate the wrong way. It's an X Y problem and that's not the kernel's fault.
The problem is a fundamental one. Nginx, like many other event loop implementations of servers, presumes that kernel code behaves consistently, precisely and correctly, instead of treating it like a black box, that sometimes lies about things or behaves completely incorrectly, it's not the first time this happened. Any correct event loop implementation should take reported poll events under advisement only and try to schedule reads and writes independently from that.
.. and the correct way to do this is SO_SNDTIMEO.
That's for blocking IO, it doesn't work on evented IO:
> [SO_RCVTIMEO and SO_SNDTIMEO] only have effect for system calls that perform socket I/O (e.g., read(2), recvmsg(2), send(2), sendmsg(2)); timeouts have no effect for select(2), poll(2), epoll_wait(2), and so on.
And from my understanding would have the same effect as a timeout on select/poll, so it's not the correct way to do anything.
> [SO_RCVTIMEO and SO_SNDTIMEO] only have effect for system calls that perform socket I/O (e.g., read(2), recvmsg(2), send(2), sendmsg(2)); timeouts have no effect for select(2), poll(2), epoll_wait(2), and so on.
And from my understanding would have the same effect as a timeout on select/poll, so it's not the correct way to do anything.
This was a unix stack problem, but it could be fixed in nginx :)
NGINX decided to have a maximum time between calls to sendfile, no matter whether the data is actually transferring or not. In configuring this setup, someone had a bad intuition about how often NGINX would end up calling sendfile.
It is a problem entirely of NGINX's devising, not a unix stack problem.
It is a problem entirely of NGINX's devising, not a unix stack problem.
You're correct, but the title is way less helpful without the "nginx". Unless the reader knows that cloudflare uses nginx, he won't know the article pertains to nginx
Is it now? nginx seems to have pretty normal behaviour here, so the core issue is write-polling buffers on linux rather than nginx itself. It seems to me nginx was the messenger of sorts, but the issue doesn't really pertain to nginx.
In a nearby comment, LoSboccacc notes that they've hit that issue [removed: on S3].
In a nearby comment, LoSboccacc notes that they've hit that issue [removed: on S3].
It sounded to me like LoSboccacc's solution was to serve files from S3, not that they experienced the same issue on S3.
You're right, I misread it.
ah! got bit by this and had to workaround serving stuff from s3. nice to see there was an actual problem and wasn't just some issue with our stack.
Related story time: At university, we ran a local quakeworld server (yes, this was in the stone age XD). And I wanted to write a tool to allow users to control to server. It would listen on a network port and pipe its input into the quakeworld server running as a child subprocess.
Sounds simple enough, right? Disregarding accept(2)ing connections,etc... it's just going to sit in an endless loop, select(2) on the sockets then read what comes in, and write that to the quakeworld server.
To debug this, everytime someone sent some data over the network, it would write a simple message ("blah") into a file on my home drive, called "~/blah" (very sophisticated debugging, I know).
Now, the semantics of select(2), cousin of poll(2), are interesting.
My understanding: select(2) blocks until you can read data from the socket.
Actual behaviour: select(2) blocks until a subsequent read on the socket does not block.
Fun fact: if there's not data available on the socket, read does not block, but returns with an error immediatly.
Result: Over the weekend, the process was spinning in an endless loop, writing 'blah' into a file on the nfs-home drive as quickly as the filesystem allowed. This being a university scale solaris NFS server, that's pretty fast.
End Result: A test file called 'blah', filled with about 800 GB of 'blah'. Good thing the admins where nice people and didn't shoot me when I went to explain the next monday why the home drive was filled with "blah" XD
EDIT: read(2) doesn't actually error on EOF, just returns 0, effect is the same though.
Sounds simple enough, right? Disregarding accept(2)ing connections,etc... it's just going to sit in an endless loop, select(2) on the sockets then read what comes in, and write that to the quakeworld server.
To debug this, everytime someone sent some data over the network, it would write a simple message ("blah") into a file on my home drive, called "~/blah" (very sophisticated debugging, I know).
Now, the semantics of select(2), cousin of poll(2), are interesting.
My understanding: select(2) blocks until you can read data from the socket.
Actual behaviour: select(2) blocks until a subsequent read on the socket does not block.
Fun fact: if there's not data available on the socket, read does not block, but returns with an error immediatly.
Result: Over the weekend, the process was spinning in an endless loop, writing 'blah' into a file on the nfs-home drive as quickly as the filesystem allowed. This being a university scale solaris NFS server, that's pretty fast.
End Result: A test file called 'blah', filled with about 800 GB of 'blah'. Good thing the admins where nice people and didn't shoot me when I went to explain the next monday why the home drive was filled with "blah" XD
EDIT: read(2) doesn't actually error on EOF, just returns 0, effect is the same though.
You have QuakeWorld and almost a terabyte in the same story. It seriously managed to write 800 gigs!? How did they have that much storage?
Wikipedia pegs QW as existing around 1996-2000...
https://en.wikipedia.org/wiki/QuakeWorld
Now, 800Gigs sounds like a lot, but I distinctly also recall that time as the era when we ran across 8GB clip issues. It wouldn't be unheard of for drives to actually be 20-60GB in size around this era, maybe larger for the expensive SCSI kind.
Getting 800GB online storage with something like RAID5 or RAID6 would probably take two fully loaded controllers; lets say 10 drives of usable storage across two arrays. (20 drives). That back of the envelope math is about 40GB per drive.
I could see this being an easy case at a LARGE university in some tech centrist area at about the timeframe of 16-18 years ago.
https://en.wikipedia.org/wiki/QuakeWorld
Now, 800Gigs sounds like a lot, but I distinctly also recall that time as the era when we ran across 8GB clip issues. It wouldn't be unheard of for drives to actually be 20-60GB in size around this era, maybe larger for the expensive SCSI kind.
Getting 800GB online storage with something like RAID5 or RAID6 would probably take two fully loaded controllers; lets say 10 drives of usable storage across two arrays. (20 drives). That back of the envelope math is about 40GB per drive.
I could see this being an easy case at a LARGE university in some tech centrist area at about the timeframe of 16-18 years ago.
Read will return 0 when the peer has closed the connection, that's how you detect it. When there is no data, the call will block instead until some data arrives (unless the socket is set to non-block mode).
Hm yeah, it could've been that I disconnected from the server when I went home for the weekend and didn't check for that in the code. It was a long time ago :)
> Good thing the admins where nice people and didn't shoot me when I went to explain the next monday why the home drive was filled with "blah" XD
They should be the ones explaining why they don't have disk quotas set up on a shared system.
They should be the ones explaining why they don't have disk quotas set up on a shared system.
AFAIR, they had one, which was so flakey they had to disable it all the time.
But yeah, maybe them being somewhat at fault as well helped me out a bit :)
But yeah, maybe them being somewhat at fault as well helped me out a bit :)
Sounds like a story straight from Unix-Haters Handbook... :).
I really wish people would stop misusing TCP resets. It's not just a fast way to close a connection. That's not what it's for at all. If you're ever thinking about sending a reset, please read RFC 793 and RFC 3360 first.
I'm looking at you Arbor.
I'm looking at you Arbor.
Well the goal here seems to be aborting a connection, not just closing it. They specifically don't want to empty the buffer, because time is up. Is that a misuse of reset?
Yes, it is a misuse of a TCP reset. The RFCs are very clear about when to use a TCP reset.
You can't use a reset just because you want to be fast or don't feel like sending any more data. Resets are for something abnormal happening. A timeout isn't abnormal to a TCP connection. It's part of the process.
You can't use a reset just because you want to be fast or don't feel like sending any more data. Resets are for something abnormal happening. A timeout isn't abnormal to a TCP connection. It's part of the process.
RFC 1122[0] says:
A TCP connection may terminate in two ways: (1) the normal TCP close sequence using a FIN handshake, and (2) an "abort" in which one or more RST segments are sent and the connection state is immediately discarded.
and
A host MAY implement a "half-duplex" TCP close sequence, so that an application that has called CLOSE cannot continue to read data from the connection. If such a host issues a CLOSE call while received data is still pending in TCP, or if new data is received after CLOSE is called, its TCP SHOULD send a RST to show that data was lost.
So this behavior seems fine unless another RFC specifically prevents it.
[0] - https://tools.ietf.org/html/rfc1122#section-4.2.2.13
A TCP connection may terminate in two ways: (1) the normal TCP close sequence using a FIN handshake, and (2) an "abort" in which one or more RST segments are sent and the connection state is immediately discarded.
and
A host MAY implement a "half-duplex" TCP close sequence, so that an application that has called CLOSE cannot continue to read data from the connection. If such a host issues a CLOSE call while received data is still pending in TCP, or if new data is received after CLOSE is called, its TCP SHOULD send a RST to show that data was lost.
So this behavior seems fine unless another RFC specifically prevents it.
[0] - https://tools.ietf.org/html/rfc1122#section-4.2.2.13
It's absolutely a misuse and causes all kinds of assumptions which the RFCs make about how people write their software to become invalid.
It's also a simple and state-free way of influencing other devices on the network. Arbor is an interesting example - when doing DDoS mitigation, an approach which doesn't require a lot of resources or statefulness is extremely useful.
On one hand, I want to agree with you: this is terrible and makes everyone else's life harder.
On the other hand, maybe its TCP's resource requirements that are at fault.
It's also a simple and state-free way of influencing other devices on the network. Arbor is an interesting example - when doing DDoS mitigation, an approach which doesn't require a lot of resources or statefulness is extremely useful.
On one hand, I want to agree with you: this is terrible and makes everyone else's life harder.
On the other hand, maybe its TCP's resource requirements that are at fault.
Closing the connection would cause the client to believe that the file has been fully downloaded - resulting in a truncated file. Not good.
HTTP does not have a way of sending an error in-mid-transfer, so a TCP reset has to suffice. The other option would be to blackhole the connection, but that would be worse for both sides.
HTTP does not have a way of sending an error in-mid-transfer, so a TCP reset has to suffice. The other option would be to blackhole the connection, but that would be worse for both sides.
> Closing the connection would cause the client to believe that the file has been fully downloaded - resulting in a truncated file. Not good.
That's not correct. Both Content-Length and the chunked encoding scheme would allow the client to know that the connection was terminated before the end of stream was reached.
That's not correct. Both Content-Length and the chunked encoding scheme would allow the client to know that the connection was terminated before the end of stream was reached.
So what are you supposed to do?
For example, Microsoft may be changing their image, but their core software is closed source. <cloud provider> may be great, but can you pull off something like this when you've got an issue? The importance of being able to debug and patch your mission critical systems is hard to overstate.
Please encourage your employer to fiscally support the open source code that they rely upon!