mirror of
https://github.com/matrix-construct/construct
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122 lines
3.6 KiB
C++
122 lines
3.6 KiB
C++
// The Construct
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//
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// Copyright (C) The Construct Developers, Authors & Contributors
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// Copyright (C) 2016-2020 Jason Volk <jason@zemos.net>
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//
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// Permission to use, copy, modify, and/or distribute this software for any
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// purpose with or without fee is hereby granted, provided that the above
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// copyright notice and this permission notice is present in all copies. The
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// full license for this software is available in the LICENSE file.
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#pragma once
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#define HAVE_IRCD_IOS_EPOLL_H
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// Forward declarations because sys/epoll.h is not included here
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extern "C"
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{
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struct epoll_event;
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}
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// This interface provides special workarounds and optimizations for the epoll
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// reactor in boost::asio on linux systems. It has to be used voluntarily by
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// the embedder of libircd, who can hook calls to epoll_wait(2), and forward
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// those calls to this interface. Our implementation then makes the syscall.
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namespace ircd::ios
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{
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using epoll_wait_proto = int (int, struct ::epoll_event *, int, int);
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template<epoll_wait_proto *>
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int epoll_wait(int, struct ::epoll_event *, int, int) noexcept;
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}
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/// This reduces the number of syscalls to epoll_wait(2), which tend to occur
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/// at the start of every epoch except in a minority of cases. These syscalls
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/// produce no ready events 99% of the time.
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///
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/// boost::asio tends to call epoll_wait(2) with timeout=0 (non-blocking) when
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/// it has more work queued that it will execute. If there's nothing queued it
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/// will set a timeout. We don't need to collect epoll events so aggressively.
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/// It's incumbent upon us to not spam thousands of non-blocking syscalls which
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/// yield no results, especially when it negates the efficiency of ircd::ctx's
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/// fast userspace context switching. We trade some responsiveness for things
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/// like asio::signal_set but gain overall performance which now has actual
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/// impact in the post-meltdown/spectre virtualized reality.
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///
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template<ircd::ios::epoll_wait_proto *_real_epoll_wait>
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[[using gnu: hot, always_inline]]
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inline int
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ircd::ios::epoll_wait(int _epfd,
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struct ::epoll_event *const _events,
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int _maxevents,
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int _timeout)
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noexcept
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{
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// Call elision tick counter.
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thread_local uint64_t tick;
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// Configured frequency to allow the call.
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const uint64_t freq
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{
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empt::freq
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};
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const bool skip
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{
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!freq || (tick < freq)
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};
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const bool peek
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{
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_timeout == 0
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};
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// Always allow blocking calls; only allow non-blocking calls which
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// satisfy our conditions.
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const bool call
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{
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!peek || !skip
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};
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// Modify tick counter prior to call, while we have the line.
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tick -= boolmask<decltype(tick)>(call) & tick;
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tick += boolmask<decltype(tick)>(!call) & 0x01;
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const int ret
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{
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call?
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_real_epoll_wait(_epfd, _events, _maxevents, _timeout): 0
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};
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// Update stats
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empt::peek += peek;
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empt::skip += !call;
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empt::call += call;
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empt::none += call && ret == 0;
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empt::result += ret & boolmask<uint>(ret >= 0);
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empt::load_low += ret >= _maxevents / 8;
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empt::load_med += ret >= _maxevents / 4;
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empt::load_high += ret >= _maxevents / 2;
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empt::load_stall += ret >= _maxevents / 1;
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if constexpr(profile::logging) if(call)
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log::logf
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{
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log, ircd::log::DEBUG,
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"EPOLL %5d tick:%lu peek:%lu skip:%lu call:%lu none:%lu result:%lu low:%lu med:%lu high:%lu stall:%lu",
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ret,
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tick,
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uint64_t(empt::peek),
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uint64_t(empt::skip),
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uint64_t(empt::call),
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uint64_t(empt::none),
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uint64_t(empt::result),
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uint64_t(empt::load_low),
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uint64_t(empt::load_med),
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uint64_t(empt::load_high),
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uint64_t(empt::load_stall),
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};
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assert(call || ret == 0);
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assert(ret <= _maxevents);
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return ret;
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}
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