Definitely agree with you that abstraction matters. I use C as an example because it's the canonical way in which we as developers can reasonably converse about directly dealing with the machine. C is still important and will continue to be important because it's arguably the best abstraction we have to write code directly against the machine. GC and STM would be orthogonal to the purpose of C.
It's really sort of pointless to argue what's possible in environments lacking real, unsafe pointers, because the whole idea with mmap()'d I/O is that it lets apps access files as if they are a region of memory. Java wraps up the behavior in FileChannel, and most other platforms have a way to access mmap() in some fashion, albeit the practical use cases are vanishingly small. In C, it's done mostly for squeezing out bits of extra performance by shrinking memory requirements/allocation cost and avoiding copies. Much of the safety mechanisms used by VM environments invalidate or make unavailable the shortcuts that are used in tandem with mapped I/O to achieve higher performance.
In the end, it's all still triggering the virtual memory interrupt that will load the page from disk if it's not in RAM, and the kernel is still going to suspend the calling thread while it's happening. No way around that. Alleviating this through thread pools would just re-introduce the issues of inter-thread locking and context switch overhead, and present an additional bottleneck; in the end just piling on extra complexity. I don't see that as a net win at all.
While the post has some great information on mapped I/O, it seems as if the author is dogmatic about the use of mapped I/O and is trying to find a way to use this hammer in a situation where it might be inappropriate.
EDIT: To the author who replied: awesome, awesome. Ultimately this is great stuff to talk about and these types of posts are what HN should really be about, even though it's turned into startup TMZ.
Broad agreement in general, but I would observe there are still use cases for non-bare-metal languages for mmap. It doesn't conflict with all VMs. Haskell could use it (it actually has surprising interfacing capabilities on this front), Google says OCaml can use mmap though AFAIK it has a less useful threading story, Lua can probably productively use it though again I'm unclear on the threading. Erlang can't use it out-of-the-box but conceptually it could be modelled as a port, though again whether you could get a performance win I don't know. Mono and the JVM could use it, though again, primitive threading story. Python and Perl have interfaces to them but you sacrifice so much performance simply by using them that yeah, it doesn't much matter. But at least in theory there are VMs that can productively use it.
Good point about C. You misunderstand the intent of my post though.
First, as a small point, I was not seriously suggesting using thread pools to implement mlock_eventfd. That's just a way that we could play around with it short of a kernel implementation, which would probably be more efficient.
Second, I don't think I'm dogmatic about mmap. It's just a cool interface that would be nice to see more widely used, and I was suggesting that it might be more usable with a small addition. Commenters here, at my blog and on Reddit have pointed out other problems, and I'm happy to learn about these. If mmap isn't good and we should keep using DIO, then I think there are still possible improvements that we could make to the interface to allow better cooperation with the kernel.
It's really sort of pointless to argue what's possible in environments lacking real, unsafe pointers, because the whole idea with mmap()'d I/O is that it lets apps access files as if they are a region of memory. Java wraps up the behavior in FileChannel, and most other platforms have a way to access mmap() in some fashion, albeit the practical use cases are vanishingly small. In C, it's done mostly for squeezing out bits of extra performance by shrinking memory requirements/allocation cost and avoiding copies. Much of the safety mechanisms used by VM environments invalidate or make unavailable the shortcuts that are used in tandem with mapped I/O to achieve higher performance.
In the end, it's all still triggering the virtual memory interrupt that will load the page from disk if it's not in RAM, and the kernel is still going to suspend the calling thread while it's happening. No way around that. Alleviating this through thread pools would just re-introduce the issues of inter-thread locking and context switch overhead, and present an additional bottleneck; in the end just piling on extra complexity. I don't see that as a net win at all.
While the post has some great information on mapped I/O, it seems as if the author is dogmatic about the use of mapped I/O and is trying to find a way to use this hammer in a situation where it might be inappropriate.
EDIT: To the author who replied: awesome, awesome. Ultimately this is great stuff to talk about and these types of posts are what HN should really be about, even though it's turned into startup TMZ.