* broadcast fd
@ 2019-10-15 17:04 Leonid Shamis
2019-10-19 1:44 ` Rich Felker
0 siblings, 1 reply; 2+ messages in thread
From: Leonid Shamis @ 2019-10-15 17:04 UTC (permalink / raw)
To: musl
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Hi Folks,
TLDR:
How do I broadcast a notification across processes, without a broker?
Long story:
I'm building an ipc-over-shm library with some unusual restrictions:
* There are multiple channels of communication (each in it's own shm).
* Each channel contains non-consumable resources (not a message queue).
* Multiple processes can produce resources on the same channel.
* Multiple consumers can access/use the resources.
* Resources are versioned and (some) old versions are kept available.
* Spurious wakeups are ok, but missing notifications (starvation) is not.
* The library must be robust (in the mutex sense), meaning a process may
crash without taking down the rest of the system.
I'm trying to figure out how to notify a process that a new resource is
available on a channel.
Right now, my library puts (the equivalent of) a robust conditional
variable in shared memory, for each channel.
Each process must spawn a thread per-channel, just to await the cv
broadcast.
I'd ideally like to use a single thread with something like an epoll.
For this, I'd need to get an fd (maybe eventfd) per channel into the hands
of every process that wants to connect to a channel.
fds, as I understand, cannot be shared via shared memory, only pipes, but
I'm required to be brokerless.
futex_fd are very deprecated.
Am I missing another approach?
Thanks,
--Leo
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^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: broadcast fd
2019-10-15 17:04 broadcast fd Leonid Shamis
@ 2019-10-19 1:44 ` Rich Felker
0 siblings, 0 replies; 2+ messages in thread
From: Rich Felker @ 2019-10-19 1:44 UTC (permalink / raw)
To: musl
On Tue, Oct 15, 2019 at 10:04:47AM -0700, Leonid Shamis wrote:
> Hi Folks,
>
> TLDR:
>
> How do I broadcast a notification across processes, without a broker?
>
> Long story:
>
> I'm building an ipc-over-shm library with some unusual restrictions:
> * There are multiple channels of communication (each in it's own shm).
> * Each channel contains non-consumable resources (not a message queue).
> * Multiple processes can produce resources on the same channel.
> * Multiple consumers can access/use the resources.
> * Resources are versioned and (some) old versions are kept available.
> * Spurious wakeups are ok, but missing notifications (starvation) is not.
> * The library must be robust (in the mutex sense), meaning a process may
> crash without taking down the rest of the system.
>
> I'm trying to figure out how to notify a process that a new resource is
> available on a channel.
>
> Right now, my library puts (the equivalent of) a robust conditional
> variable in shared memory, for each channel.
> Each process must spawn a thread per-channel, just to await the cv
> broadcast.
>
> I'd ideally like to use a single thread with something like an epoll.
> For this, I'd need to get an fd (maybe eventfd) per channel into the hands
> of every process that wants to connect to a channel.
> fds, as I understand, cannot be shared via shared memory, only pipes, but
> I'm required to be brokerless.
> futex_fd are very deprecated.
>
> Am I missing another approach?
If you want the events associated with the shared memory objects, and
you have file descriptors for these objects (they're POSIX shm or
hand-rolled tmpfs shm, as opposed to sysvipc), perhaps there's a way
to convey the event availability through the fd. For example is there
a way you could use inotify to do it?
Otherwise I think you already have the best way. Keep in mind the
cv-waiting thread can be made tiny if you use a minimal stack size,
run with signals blocked, and just have it convert cv events to
self-pipe/eventfd/etc.
Rich
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