How do asynchronous calls à la `epoll` or `kqueue` work at the CPU (assembly) level?
I've recently been trying to wrap my brain around asynchronous I/O, that is, epoll() on Linux and kqueue() on FreeBSD; instead of creating multiple threads and using something like select(), you can create only one thread and still respond to many I/O events at once.
I've been trying to understand how this works at a deep level in the machine code, but have hit a brick wall. (Perhaps my Google-Fu is just lacking.) I can't seem to find any CPU instructions related to async I/O at all - so how does it work?
assembly x86-64 epoll kqueue
asked Nov 22 '18 at 9:11
Fredrick BrennanFredrick Brennan
4,41011742
add a comment |
I've recently been trying to wrap my brain around asynchronous I/O, that is, epoll() on Linux and kqueue() on FreeBSD; instead of creating multiple threads and using something like select(), you can create only one thread and still respond to many I/O events at once.
I've been trying to understand how this works at a deep level in the machine code, but have hit a brick wall. (Perhaps my Google-Fu is just lacking.) I can't seem to find any CPU instructions related to async I/O at all - so how does it work?
assembly x86-64 epoll kqueue
asked Nov 22 '18 at 9:11
Fredrick BrennanFredrick Brennan
4,41011742
not sure if I understand correctly what you mean by "asynchronous I/O", I'm afraid on assembly level it could lead one to completely different things than when used in context of epoll/kqueue, which seems to me more like queues with kernel/user boundary crossing (and the content like file handle can be also pretty much anything, the particular I/O done on it, or the event triggering its inclusion into queue can be anything, and if we are talking about block devices, it's probably up to the driver to define those). So the inner implementation of the queues in assembly is probably likemovs...
– Ped7g
Nov 22 '18 at 9:55
3
epoll,kqueueandselectall do the same thing, they wait for events, just with different interfaces. Also none of them asynchronous calls themselves, but can be used to wait for events that result from asynchronous calls.
– Ross Ridge
Nov 22 '18 at 10:35
1
There is nothing special going on on the machine code level. The operating system does the IO at some time and you can ask if it's done yet.
– fuz
Nov 22 '18 at 12:51
1
Fundamentally, asynchronous I/O is done with interrupts.
– EOF
Nov 22 '18 at 18:52
add a comment |
I've recently been trying to wrap my brain around asynchronous I/O, that is, epoll() on Linux and kqueue() on FreeBSD; instead of creating multiple threads and using something like select(), you can create only one thread and still respond to many I/O events at once.
I've been trying to understand how this works at a deep level in the machine code, but have hit a brick wall. (Perhaps my Google-Fu is just lacking.) I can't seem to find any CPU instructions related to async I/O at all - so how does it work?
assembly x86-64 epoll kqueue
asked Nov 22 '18 at 9:11
Fredrick BrennanFredrick Brennan
4,41011742
I've recently been trying to wrap my brain around asynchronous I/O, that is, epoll() on Linux and kqueue() on FreeBSD; instead of creating multiple threads and using something like select(), you can create only one thread and still respond to many I/O events at once.
I've been trying to understand how this works at a deep level in the machine code, but have hit a brick wall. (Perhaps my Google-Fu is just lacking.) I can't seem to find any CPU instructions related to async I/O at all - so how does it work?
assembly x86-64 epoll kqueue
assembly x86-64 epoll kqueue
asked Nov 22 '18 at 9:11
Fredrick BrennanFredrick Brennan
4,41011742
asked Nov 22 '18 at 9:11
Fredrick BrennanFredrick Brennan
4,41011742
asked Nov 22 '18 at 9:11
Fredrick BrennanFredrick Brennan
4,41011742
asked Nov 22 '18 at 9:11
Fredrick BrennanFredrick Brennan
4,41011742
asked Nov 22 '18 at 9:11
Fredrick BrennanFredrick Brennan
4,41011742
4,41011742
not sure if I understand correctly what you mean by "asynchronous I/O", I'm afraid on assembly level it could lead one to completely different things than when used in context of epoll/kqueue, which seems to me more like queues with kernel/user boundary crossing (and the content like file handle can be also pretty much anything, the particular I/O done on it, or the event triggering its inclusion into queue can be anything, and if we are talking about block devices, it's probably up to the driver to define those). So the inner implementation of the queues in assembly is probably likemovs...
– Ped7g
Nov 22 '18 at 9:55
3
epoll,kqueueandselectall do the same thing, they wait for events, just with different interfaces. Also none of them asynchronous calls themselves, but can be used to wait for events that result from asynchronous calls.
– Ross Ridge
Nov 22 '18 at 10:35
1
There is nothing special going on on the machine code level. The operating system does the IO at some time and you can ask if it's done yet.
– fuz
Nov 22 '18 at 12:51
1
Fundamentally, asynchronous I/O is done with interrupts.
– EOF
Nov 22 '18 at 18:52
add a comment |
not sure if I understand correctly what you mean by "asynchronous I/O", I'm afraid on assembly level it could lead one to completely different things than when used in context of epoll/kqueue, which seems to me more like queues with kernel/user boundary crossing (and the content like file handle can be also pretty much anything, the particular I/O done on it, or the event triggering its inclusion into queue can be anything, and if we are talking about block devices, it's probably up to the driver to define those). So the inner implementation of the queues in assembly is probably likemovs...
– Ped7g
Nov 22 '18 at 9:55
3
epoll,kqueueandselectall do the same thing, they wait for events, just with different interfaces. Also none of them asynchronous calls themselves, but can be used to wait for events that result from asynchronous calls.
– Ross Ridge
Nov 22 '18 at 10:35
1
There is nothing special going on on the machine code level. The operating system does the IO at some time and you can ask if it's done yet.
– fuz
Nov 22 '18 at 12:51
1
Fundamentally, asynchronous I/O is done with interrupts.
– EOF
Nov 22 '18 at 18:52
not sure if I understand correctly what you mean by "asynchronous I/O", I'm afraid on assembly level it could lead one to completely different things than when used in context of epoll/kqueue, which seems to me more like queues with kernel/user boundary crossing (and the content like file handle can be also pretty much anything, the particular I/O done on it, or the event triggering its inclusion into queue can be anything, and if we are talking about block devices, it's probably up to the driver to define those). So the inner implementation of the queues in assembly is probably like
movs...– Ped7g
Nov 22 '18 at 9:55
not sure if I understand correctly what you mean by "asynchronous I/O", I'm afraid on assembly level it could lead one to completely different things than when used in context of epoll/kqueue, which seems to me more like queues with kernel/user boundary crossing (and the content like file handle can be also pretty much anything, the particular I/O done on it, or the event triggering its inclusion into queue can be anything, and if we are talking about block devices, it's probably up to the driver to define those). So the inner implementation of the queues in assembly is probably like
movs...– Ped7g
Nov 22 '18 at 9:55
3
3
epoll, kqueue and select all do the same thing, they wait for events, just with different interfaces. Also none of them asynchronous calls themselves, but can be used to wait for events that result from asynchronous calls.– Ross Ridge
Nov 22 '18 at 10:35
epoll, kqueue and select all do the same thing, they wait for events, just with different interfaces. Also none of them asynchronous calls themselves, but can be used to wait for events that result from asynchronous calls.– Ross Ridge
Nov 22 '18 at 10:35
1
1
There is nothing special going on on the machine code level. The operating system does the IO at some time and you can ask if it's done yet.
– fuz
Nov 22 '18 at 12:51
There is nothing special going on on the machine code level. The operating system does the IO at some time and you can ask if it's done yet.
– fuz
Nov 22 '18 at 12:51
1
1
Fundamentally, asynchronous I/O is done with interrupts.
– EOF
Nov 22 '18 at 18:52
Fundamentally, asynchronous I/O is done with interrupts.
– EOF
Nov 22 '18 at 18:52
add a comment |
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not sure if I understand correctly what you mean by "asynchronous I/O", I'm afraid on assembly level it could lead one to completely different things than when used in context of epoll/kqueue, which seems to me more like queues with kernel/user boundary crossing (and the content like file handle can be also pretty much anything, the particular I/O done on it, or the event triggering its inclusion into queue can be anything, and if we are talking about block devices, it's probably up to the driver to define those). So the inner implementation of the queues in assembly is probably like
movs...– Ped7g
Nov 22 '18 at 9:55
3
epoll,kqueueandselectall do the same thing, they wait for events, just with different interfaces. Also none of them asynchronous calls themselves, but can be used to wait for events that result from asynchronous calls.– Ross Ridge
Nov 22 '18 at 10:35
1
There is nothing special going on on the machine code level. The operating system does the IO at some time and you can ask if it's done yet.
– fuz
Nov 22 '18 at 12:51
1
Fundamentally, asynchronous I/O is done with interrupts.
– EOF
Nov 22 '18 at 18:52