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Marc Alexander Lehmann 2 years ago
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f774e9d2e2
  1. 38
      ev_iouring.c

38
ev_iouring.c

@ -46,11 +46,10 @@
* of linux aio or epoll and so on and so on. and you could do event stuff
* without any syscalls. what's not to like?
* d) ok, it's vastly more complex, but that's ok, really.
* e) why 3 mmaps instead of one? one would be more space-efficient,
* and I can't see what benefit three would have (other than being
* e) why two mmaps instead of one? one would be more space-efficient,
* and I can't see what benefit two would have (other than being
* somehow resizable/relocatable, but that's apparently not possible).
* (FIXME: newer kernels can use 2 mmaps only, need to look into this).
* f) hmm, it's practiclaly undebuggable (gdb can't access the memory, and
* f) hmm, it's practically undebuggable (gdb can't access the memory, and
* the bizarre way structure offsets are communicated makes it hard to
* just print the ring buffer heads, even *iff* the memory were visible
* in gdb. but then, that's also ok, really.
@ -61,25 +60,22 @@
* like a µ-optimisation by the io_uring author for his personal
* applications, to the detriment of everybody else who just wants
* an event loop. but, umm, ok, if that's all, it could be worse.
* (FIXME: jens mentioned timeout commands, need to investigate)
* h) there is a hardcoded limit of 4096 outstanding events. okay,
* at least there is no arbitrary low system-wide limit...
* (FIXME: apparently, this was increased to 32768 in later kernels(
* (from what I gather form Jens Axboe, it simply didn't occur to him,
* and he made good on it by adding an unlimited nuber of timeouts
* later :).
* h) initially there was a hardcoded limit of 4096 outstanding events.
* later versions not onlyx bump this to 32k, but also can handle
* an unlimited amount of events, so this only affects the batch size.
* i) unlike linux aio, you *can* register more then the limit
* of fd events, and the kernel will "gracefully" signal an
* overflow, after which you could destroy and recreate the kernel
* state, a bit bigger, or fall back to e.g. poll. thats not
* totally insane, but kind of questions the point a high
* performance I/O framework when it doesn't really work
* under stress.
* (FIXME: iouring should no longer drop events, need to investigate)
* j) but, oh my! is has exactly the same bugs as the linux aio backend,
* where some undocumented poll combinations just fail.
* so we need epoll AGAIN as a fallback. AGAIN! epoll!! and of course,
* this is completely undocumented, have I mantioned this already?
* of fd events. while early verisons of io_uring signalled an overflow
* and you ended up getting wet. 5.5+ does not do this anymore.
* j) but, oh my! it had exactly the same bugs as the linux aio backend,
* where some undocumented poll combinations just fail. fortunately,
* after finally reaching the author, he was more than willing to fix
* this probably in 5.6+.
* k) overall, the *API* itself is, I dare to say, not a total trainwreck.
* the big isuess with it are the bugs requiring epoll, which might
* or might not get fixed (do I hold my breath?).
* once the bugs ae fixed (probably in 5.6+), it will be without
* competition.
*/
/* TODO: use internal TIMEOUT */

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