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master
Marc Alexander Lehmann 2019-06-25 04:52:30 +00:00
parent 34c32ad678
commit 26f14e7c9b
5 changed files with 174 additions and 129 deletions
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8
Changes
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@ -1,7 +1,11 @@
Revision history for libev, a high-performance and full-featured event loop.
4.26 (no standalone release)
- update to libec 0x00010006.
- linux aio backend almost complete rewritten to work around its
limitations.
- epoll backend now mandatory for linux aio backend.
4.26 (EV only)
- update to libecb 0x00010006.
- new experimental linux aio backend (linux 4.18+).
- removed redundant 0-ptr check in ev_once.
- updated/extended ev_set_allocator documentation.

2
ev.c
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@ -2149,7 +2149,7 @@ fd_reify (EV_P)
unsigned char o_events = anfd->events;
unsigned char o_reify = anfd->reify;
anfd->reify = 0;
anfd->reify = 0;
/*if (expect_true (o_reify & EV_ANFD_REIFY)) probably a deoptimisation */
{

290
ev_linuxaio.c
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@ -37,32 +37,70 @@
* either the BSD or the GPL.
*/
#define EPOLL_FALLBACK 1
/*
* general notes about linux aio:
*
* a) at first, the linux aio IOCB_CMD_POLL functionality introduced in
* 4.18 looks too good to be true: both watchers and events can be
* batched, and events can even be handled in userspace using
* a ring buffer shared with the kernel. watchers can be canceled
* regardless of whether the fd has been closed. no problems with fork.
* ok, the ring buffer is 200% undocumented (there isn't even a
* header file), but otherwise, it's pure bliss!
* b) ok, watchers are one-shot, so you have to re-arm active ones
* on every iteration. so much for syscall-less event handling,
* but at least these re-arms can be batched, no big deal, right?
* c) well, linux as usual: the documentation lies to you: io_submit
* sometimes returns EINVAL because the kernel doesn't feel like
* handling your poll mask - ttys can be polled for POLLOUT,
* POLLOUT|POLLIN, but polling for POLLIN fails. just great,
* so we have to fall back to something else (hello, epoll),
* but at least the fallback can be slow, because these are
* exceptional cases, right?
* d) hmm, you have to tell the kernel the maximum number of watchers
* you want to queue when initialiasing the aio context. but of
* course the real limit is magically calculated in the kernel, and
* is often higher then we asked for. so we just have to destroy
* the aio context and re-create it a bit larger if we hit the limit.
* (starts to remind you of epoll? well, it's a bit more deterministic
* and less gambling, but still ugly as hell).
* e) that's when you find out you can also hit an arbitrary system-wide
* limit. or the kernel simply doesn't want to handle your watchers.
* what the fuck do we do then? you guessed it, in the middle
* of event handling we have to switch to 100% epoll polling. and
* that better is as fast as normal epoll polling, so you practically
* have to use the normal epoll backend with all its quirks.
* f) end result of this trainwreck: it inherits all the disadvantages
* from epoll, while adding a number on its own. why even bother to use
* it? because if conditions are right and your fds are supported and you
* don't hit a limit, this backend is actually faster, doesn't gamble with
* your fds, batches watchers and events and doesn't require costly state
* recreates. well, until it does.
* g) all of this makes this backend use almost twice as much code as epoll.
* which in turn uses twice as much code as poll. and thats not counting
* the fact that this backend also depends on the epoll backend, making
* it three times as much code as poll, or kqueue.
* h) bleah. why can't linux just do kqueue. sure kqueue is ugly, but by now
* it's clear that whwetaver linux does is far, far far worse.
*/
#include <sys/time.h> /* actually linux/time.h, but we must assume they are compatible */
#include <poll.h>
#include <linux/aio_abi.h>
#if EPOLL_FALLBACK
# include <sys/epoll.h>
#endif
/* we try to fill 4kB pages exactly.
* the ring buffer header is 32 bytes, every io event is 32 bytes.
* the kernel takes the io event number, doubles it, adds 2, adds the ring buffer.
* therefore the calculation below will use "exactly" 4kB for the ring buffer
*/
#define EV_LINUXAIO_DEPTH (128 / 2 - 2 - 1) /* max. number of io events per batch */
/*****************************************************************************/
/* syscall wrapdadoop - this section has the raw syscall definitions */
/* syscall wrapdadoop - this section has the raw api/abi definitions */
#include <sys/syscall.h> /* no glibc wrappers */
/* aio_abi.h is not versioned in any way, so we cannot test for its existance */
#define IOCB_CMD_POLL 5
/* taken from linux/fs/aio.c */
/* taken from linux/fs/aio.c. yup, that's a .c file.
* not only is this totally undocumented, not even the source code
* can tell you what the future semantics of compat_features and
* incompat_features are, or what header_length actually is for.
*/
#define AIO_RING_MAGIC 0xa10a10a1
#define AIO_RING_INCOMPAT_FEATURES 0
struct aio_ring
@ -118,6 +156,42 @@ evsys_io_getevents (aio_context_t ctx_id, long min_nr, long nr, struct io_event
/*****************************************************************************/
/* actual backed implementation */
ecb_cold
static int
linuxaio_nr_events (EV_P)
{
/* we start with 16 iocbs and incraese from there
* that's tiny, but the kernel has a rather low system-wide
* limit that can be reached quickly, so let's be parsimonious
* with this resource.
* Rest assured, the kernel generously rounds up small and big numbers
* in different ways (but doesn't seem to charge you for it).
* The 15 here is because the kernel usually has a power of two as aio-max-nr,
* and this helps to take advantage of that limit.
*/
/* we try to fill 4kB pages exactly.
* the ring buffer header is 32 bytes, every io event is 32 bytes.
* the kernel takes the io requests number, doubles it, adds 2
* and adds the ring buffer.
* the way we use this is by starting low, and then roughly doubling the
* size each time we hit a limit.
*/
int requests = 15 << linuxaio_iteration;
int one_page = (4096
/ sizeof (struct io_event) ) / 2; /* how many fit into one page */
int first_page = ((4096 - sizeof (struct aio_ring))
/ sizeof (struct io_event) - 2) / 2; /* how many fit into the first page */
/* if everything fits into one page, use count exactly */
if (requests > first_page)
/* otherwise, round down to full pages and add the first page */
requests = requests / one_page * one_page + first_page;
return requests;
}
/* we use out own wrapper structure in acse we ever want to do something "clever" */
typedef struct aniocb
{
@ -163,16 +237,17 @@ linuxaio_modify (EV_P_ int fd, int oev, int nev)
array_needsize (ANIOCBP, linuxaio_iocbps, linuxaio_iocbpmax, fd + 1, linuxaio_array_needsize_iocbp);
ANIOCBP iocb = linuxaio_iocbps [fd];
#if EPOLL_FALLBACK
if (iocb->io.aio_reqprio < 0)
{
/* we handed this fd over to epoll, so undo this first */
/* we do it manually becvause the optimisations on epoll_modfy won't do us any good */
epoll_ctl (backend_fd, EPOLL_CTL_DEL, fd, 0);
iocb->io.aio_reqprio = 0;
}
#endif
if (iocb->io.aio_buf)
evsys_io_cancel (linuxaio_ctx, &iocb->io, (struct io_event *)0); /* always returns an error relevant kernels */
/* io_cancel always returns some error on relevant kernels, but works */
evsys_io_cancel (linuxaio_ctx, &iocb->io, (struct io_event *)0);
if (nev)
{
@ -188,57 +263,19 @@ linuxaio_modify (EV_P_ int fd, int oev, int nev)
}
}
#if EPOLL_FALLBACK
static void
linuxaio_rearm_epoll (EV_P_ struct iocb *iocb, int op)
{
struct epoll_event eev;
eev.events = EPOLLONESHOT;
if (iocb->aio_buf & POLLIN ) eev.events |= EPOLLIN ;
if (iocb->aio_buf & POLLOUT) eev.events |= EPOLLOUT;
eev.data.fd = iocb->aio_fildes;
if (epoll_ctl (backend_fd, op, iocb->aio_fildes, &eev) < 0)
ev_syserr ("(libeio) linuxaio epoll_ctl");
}
static void
linuxaio_epoll_cb (EV_P_ struct ev_io *w, int revents)
{
struct epoll_event events[16];
for (;;)
{
int idx;
int res = epoll_wait (backend_fd, events, sizeof (events) / sizeof (events [0]), 0);
if (expect_false (res < 0))
ev_syserr ("(libev) linuxaio epoll_wait");
else if (!res)
break;
for (idx = res; idx--; )
{
int fd = events [idx].data.fd;
uint32_t ev = events [idx].events;
assert (("libev: iocb fd must be in-bounds", fd >= 0 && fd < anfdmax));
linuxaio_rearm_epoll (EV_A_ &linuxaio_iocbps [fd]->io, EPOLL_CTL_MOD);
fd_event (EV_A_ fd,
(ev & (EPOLLOUT | EPOLLERR | EPOLLHUP) ? EV_WRITE : 0)
| (ev & (EPOLLIN | EPOLLERR | EPOLLHUP) ? EV_READ : 0));
}
if (res < sizeof (events) / sizeof (events [0]))
break;
}
epoll_poll (EV_A_ 0);
}
#endif
static void
linuxaio_fd_rearm (EV_P_ int fd)
{
anfds [fd].events = 0;
linuxaio_iocbps [fd]->io.aio_buf = 0;
fd_change (EV_A_ fd, EV_ANFD_REIFY);
}
static void
linuxaio_parse_events (EV_P_ struct io_event *ev, int nr)
@ -250,11 +287,6 @@ linuxaio_parse_events (EV_P_ struct io_event *ev, int nr)
assert (("libev: iocb fd must be in-bounds", fd >= 0 && fd < anfdmax));
/* linux aio is oneshot: rearm fd. TODO: this does more work than needed */
linuxaio_iocbps [fd]->io.aio_buf = 0;
anfds [fd].events = 0;
fd_change (EV_A_ fd, 0);
/* feed events, we do not expect or handle POLLNVAL */
fd_event (
EV_A_
@ -263,6 +295,9 @@ linuxaio_parse_events (EV_P_ struct io_event *ev, int nr)
| (res & (POLLIN | POLLERR | POLLHUP) ? EV_READ : 0)
);
/* linux aio is oneshot: rearm fd. TODO: this does more work than needed */
linuxaio_fd_rearm (EV_A_ fd);
--nr;
++ev;
}
@ -348,6 +383,13 @@ linuxaio_get_events (EV_P_ ev_tstamp timeout)
}
}
static int
linuxaio_io_setup (EV_P)
{
linuxaio_ctx = 0;
return evsys_io_setup (linuxaio_nr_events (EV_A), &linuxaio_ctx);
}
static void
linuxaio_poll (EV_P_ ev_tstamp timeout)
{
@ -360,38 +402,10 @@ linuxaio_poll (EV_P_ ev_tstamp timeout)
/* which allows us to pinpoint the errornous iocb */
for (submitted = 0; submitted < linuxaio_submitcnt; )
{
#if 0
int res;
if (linuxaio_submits[submitted]->aio_fildes == backend_fd)
res = evsys_io_submit (linuxaio_ctx, 1, linuxaio_submits + submitted);
else
{ res = -1; errno = EINVAL; };
#else
int res = evsys_io_submit (linuxaio_ctx, linuxaio_submitcnt - submitted, linuxaio_submits + submitted);
#endif
if (expect_false (res < 0))
if (errno == EAGAIN)
{
/* This happens when the ring buffer is full, at least. I assume this means
* that the event was queued synchronously during io_submit, and thus
* the buffer overflowed.
* In this case, we just try in next loop iteration.
* This should not result in a few fds taking priority, as the interface
* is one-shot, and we submit iocb's in a round-robin fashion.
* TODO: maybe make "submitted" persistent, so we don't have to memmove?
*/
if (ecb_expect_false (submitted))
{
memmove (linuxaio_submits, linuxaio_submits + submitted, (linuxaio_submitcnt - submitted) * sizeof (*linuxaio_submits));
linuxaio_submitcnt -= submitted;
}
timeout = 0;
break;
}
#if EPOLL_FALLBACK
else if (errno == EINVAL)
if (errno == EINVAL)
{
/* This happens for unsupported fds, officially, but in my testing,
* also randomly happens for supported fds. We fall back to good old
@ -401,13 +415,48 @@ linuxaio_poll (EV_P_ ev_tstamp timeout)
* fails but POLLIN|POLLOUT works.
*/
struct iocb *iocb = linuxaio_submits [submitted];
linuxaio_rearm_epoll (EV_A_ linuxaio_submits [submitted], EPOLL_CTL_ADD);
epoll_modify (EV_A_ iocb->aio_fildes, 0, anfds [iocb->aio_fildes].events);
iocb->aio_reqprio = -1; /* mark iocb as epoll */
res = 1; /* skip this iocb */
res = 1; /* skip this iocb - another iocb, another chance */
}
else if (errno == EAGAIN)
{
/* This happens when the ring buffer is full, or some other shit we
* dont' know and isn't documented. Most likely because we have too
* many requests and linux aio can't be assed to handle them.
* In this case, we try to allocate a larger ring buffer, freeing
* ours first. This might fail, in which case we have to fall back to 100%
* epoll.
* God, how I hate linux not getting its act together. Ever.
*/
evsys_io_destroy (linuxaio_ctx);
linuxaio_submitcnt = 0;
/* rearm all fds with active iocbs */
{
int fd;
for (fd = 0; fd < linuxaio_iocbpmax; ++fd)
if (linuxaio_iocbps [fd]->io.aio_buf)
linuxaio_fd_rearm (EV_A_ fd);
}
++linuxaio_iteration;
if (linuxaio_io_setup (EV_A) < 0)
{
/* to bad, we can't get a new aio context, go 100% epoll */
linuxaio_free_iocbp (EV_A);
ev_io_stop (EV_A_ &linuxaio_epoll_w);
ev_ref (EV_A);
linuxaio_ctx = 0;
backend_modify = epoll_modify;
backend_poll = epoll_poll;
}
timeout = 0;
/* it's easiest to handle this mess in another iteration */
return;
}
#endif
else if (errno == EBADF)
{
fd_kill (EV_A_ linuxaio_submits [submitted]->aio_fildes);
@ -433,25 +482,18 @@ linuxaio_init (EV_P_ int flags)
{
/* would be great to have a nice test for IOCB_CMD_POLL instead */
/* also: test some semi-common fd types, such as files and ttys in recommended_backends */
#if EPOLL_FALLBACK
/* 4.19 made epoll work */
/* 4.18 introduced IOCB_CMD_POLL, 4.19 made epoll work */
if (ev_linux_version () < 0x041300)
return 0;
#else
/* 4.18 introduced IOCB_CMD_POLL */
if (ev_linux_version () < 0x041200)
return 0;
#endif
linuxaio_ctx = 0;
if (evsys_io_setup (EV_LINUXAIO_DEPTH, &linuxaio_ctx) < 0)
if (!epoll_init (EV_A_ 0))
return 0;
#if EPOLL_FALLBACK
backend_fd = ev_epoll_create ();
if (backend_fd < 0)
linuxaio_iteration = 0;
if (linuxaio_io_setup (EV_A) < 0)
{
evsys_io_destroy (linuxaio_ctx);
epoll_destroy (EV_A);
return 0;
}
@ -459,7 +501,6 @@ linuxaio_init (EV_P_ int flags)
ev_set_priority (&linuxaio_epoll_w, EV_MAXPRI);
ev_io_start (EV_A_ &linuxaio_epoll_w);
ev_unref (EV_A); /* watcher should not keep loop alive */
#endif
backend_modify = linuxaio_modify;
backend_poll = linuxaio_poll;
@ -478,9 +519,7 @@ inline_size
void
linuxaio_destroy (EV_P)
{
#if EPOLL_FALLBACK
close (backend_fd);
#endif
epoll_destroy (EV_A);
linuxaio_free_iocbp (EV_A);
evsys_io_destroy (linuxaio_ctx);
}
@ -493,19 +532,18 @@ linuxaio_fork (EV_P)
linuxaio_destroy (EV_A);
linuxaio_submitcnt = 0; /* all pointers were invalidated */
linuxaio_ctx = 0;
while (evsys_io_setup (EV_LINUXAIO_DEPTH, &linuxaio_ctx) < 0)
linuxaio_iteration = 0; /* we start over in the child */
while (linuxaio_io_setup (EV_A) < 0)
ev_syserr ("(libev) linuxaio io_setup");
#if EPOLL_FALLBACK
while ((backend_fd = ev_epoll_create ()) < 0)
ev_syserr ("(libev) linuxaio epoll_create");
epoll_fork (EV_A);
ev_io_stop (EV_A_ &linuxaio_epoll_w);
ev_io_init (EV_A_ &linuxaio_epoll_w, linuxaio_epoll_cb, backend_fd, EV_READ);
ev_io_set (EV_A_ &linuxaio_epoll_w, backend_fd, EV_READ);
ev_io_start (EV_A_ &linuxaio_epoll_w);
#endif
fd_rearm_all (EV_A);
/* epoll_fork already did this. hopefully */
/*fd_rearm_all (EV_A);*/
}

1
ev_vars.h
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@ -109,6 +109,7 @@ VARx(int, epoll_epermmax)
#if EV_USE_LINUXAIO || EV_GENWRAP
VARx(aio_context_t, linuxaio_ctx)
VARx(int, linuxaio_iteration)
VARx(struct aniocb **, linuxaio_iocbps)
VARx(int, linuxaio_iocbpmax)
VARx(struct iocb **, linuxaio_submits)

2
ev_wrap.h
View File

@ -54,6 +54,7 @@
#define linuxaio_epoll_w ((loop)->linuxaio_epoll_w)
#define linuxaio_iocbpmax ((loop)->linuxaio_iocbpmax)
#define linuxaio_iocbps ((loop)->linuxaio_iocbps)
#define linuxaio_iteration ((loop)->linuxaio_iteration)
#define linuxaio_submitcnt ((loop)->linuxaio_submitcnt)
#define linuxaio_submitmax ((loop)->linuxaio_submitmax)
#define linuxaio_submits ((loop)->linuxaio_submits)
@ -160,6 +161,7 @@
#undef linuxaio_epoll_w
#undef linuxaio_iocbpmax
#undef linuxaio_iocbps
#undef linuxaio_iteration
#undef linuxaio_submitcnt
#undef linuxaio_submitmax
#undef linuxaio_submits