Caveats of select/poll vs. epoll reactors in Twisted

Question

Everything I've read and experienced ( Tornado based apps ) leads me to believe that ePoll is a natural replacement for Select and Poll based networking, especially with Twisted. Which makes me paranoid, its pretty rare for a better technique or methodology not to come with a price.

Reading a couple dozen comparisons between epoll and alternatives shows that epoll is clearly the champion for speed and scalability, specifically that it scales in a linear fashion which is fantastic. That said, what about processor and memory utilization, is epoll still the champ?

Answer 1

For very small numbers of sockets (varies depending on your hardware, of course, but we're talking about something on the order of 10 or fewer), select can beat epoll in memory usage and runtime speed. Of course, for such small numbers of sockets, both mechanisms are so fast that you don't really care about this difference in the vast majority of cases.

One clarification, though. Both select and epoll scale linearly. A big difference, though, is that the userspace-facing APIs have complexities that are based on different things. The cost of a select call goes roughly with the value of the highest numbered file descriptor you pass it. If you select on a single fd, 100, then that's roughly twice as expensive as selecting on a single fd, 50. Adding more fds below the highest isn't quite free, so it's a little more complicated than this in practice, but this is a good first approximation for most implementations.

The cost of epoll is closer to the number of file descriptors that actually have events on them. If you're monitoring 200 file descriptors, but only 100 of them have events on them, then you're (very roughly) only paying for those 100 active file descriptors. This is where epoll tends to offer one of its major advantages over select. If you have a thousand clients that are mostly idle, then when you use select you're still paying for all one thousand of them. However, with epoll, it's like you've only got a few - you're only paying for the ones that are active at any given time.

All this means that epoll will lead to less CPU usage for most workloads. As far as memory usage goes, it's a bit of a toss up. select does manage to represent all the necessary information in a highly compact way (one bit per file descriptor). And the FD_SETSIZE (typically 1024) limitation on how many file descriptors you can use with select means that you'll never spend more than 128 bytes for each of the three fd sets you can use with select (read, write, exception). Compared to those 384 bytes max, epoll is sort of a pig. Each file descriptor is represented by a multi-byte structure. However, in absolute terms, it's still not going to use much memory. You can represent a huge number of file descriptors in a few dozen kilobytes (roughly 20k per 1000 file descriptors, I think). And you can also throw in the fact that you have to spend all 384 of those bytes with select if you only want to monitor one file descriptor but its value happens to be 1024, wheras with epoll you'd only spend 20 bytes. Still, all these numbers are pretty small, so it doesn't make much difference.

And there's also that other benefit of epoll, which perhaps you're already aware of, that it is not limited to FD_SETSIZE file descriptors. You can use it to monitor as many file descriptors as you have. And if you only have one file descriptor, but its value is greater than FD_SETSIZE, epoll works with that too, but select does not.

Randomly, I've also recently discovered one slight drawback to epoll as compared to select or poll. While none of these three APIs supports normal files (ie, files on a file system), select and poll present this lack of support as reporting such descriptors as always readable and always writeable. This makes them unsuitable for any meaningful kind of non-blocking filesystem I/O, a program which uses select or poll and happens to encounter a file descriptor from the filesystem will at least continue to operate (or if it fails, it won't be because of select or poll), albeit it perhaps not with the best performance.

On the other hand, epoll will fail fast with an error (EPERM, apparently) when asked to monitor such a file descriptor. Strictly speaking, this is hardly incorrect. It's merely signalling its lack of support in an explicit way. Normally I would applaud explicit failure conditions, but this one is undocumented (as far as I can tell) and results in a completely broken application, rather than one which merely operates with potentially degraded performance.

In practice, the only place I've seen this come up is when interacting with stdio. A user might redirect stdin or stdout from/to a normal file. Whereas previously stdin and stdout would have been a pipe -- supported by epoll just fine -- it then becomes a normal file and epoll fails loudly, breaking the application.

Answer 2

In tests at my company, one issue with epoll() came up, thus a single cost compared to select.

When attempting to read from the network with a timeout, creating an epoll_fd ( instead of a FD_SET ), and adding the fd to the epoll_fd, is much more expensive than creating a FD_SET (which is a simple malloc).

As per the previous answer, as the number of FDs in the process becomes large, the cost of select() becomes higher, but in our testing, even with fd values in the 10,000's, select was still a winner. These are cases where there is only one fd that a thread is waiting on, and simply trying to overcome the fact that network read, and network write, doesn't timeout when using a blocking thread model. Of course, blocking thread models are low performance compared to non-blocking reactor systems, but there are occasions where, to integrate with a particular legacy code base, it is required.

This kind of use case is rare in high performance applications, because a reactor model doesn't need to create a new epoll_fd every time. For the model where an epoll_fd is long-lived --- which is clearly preferred for any high performance server design --- epoll is the clear winner in every way.