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/**
* @file coro_scheduler.cpp
* @author Nat Goodspeed
* @date 2024-08-05
* @brief Implementation for llcoro::scheduler.
*
* $LicenseInfo:firstyear=2024&license=viewerlgpl$
* Copyright (c) 2024, Linden Research, Inc.
* $/LicenseInfo$
*/
// Precompiled header
#include "linden_common.h"
// associated header
#include "coro_scheduler.h"
// STL headers
// std headers
#include <iomanip>
// external library headers
#include <boost/fiber/operations.hpp>
// other Linden headers
#include "llcallbacklist.h"
#include "llcoros.h"
#include "lldate.h"
#include "llerror.h"
namespace llcoro
{
const F64 scheduler::DEFAULT_TIMESLICE{ LL::Timers::DEFAULT_TIMESLICE };
const std::string qname("General");
scheduler::scheduler():
// Since use_scheduling_algorithm() must be called before any other
// Boost.Fibers operations, we can assume that the calling fiber is in
// fact the main fiber.
mMainID(boost::this_fiber::get_id()),
mStart(LLDate::now().secondsSinceEpoch()),
mQueue(LL::WorkQueue::getInstance(qname))
{}
void scheduler::awakened( boost::fibers::context* ctx) noexcept
{
if (ctx->get_id() == mMainID)
{
// If the fiber that just came ready is the main fiber, record its
// pointer.
llassert(! mMainCtx);
mMainCtx = ctx;
}
// Delegate to round_robin::awakened() as usual, even for the main fiber.
// This way, as long as other fibers don't take too long, we can just let
// normal round_robin processing pass control to the main fiber.
super::awakened(ctx);
}
boost::fibers::context* scheduler::pick_next() noexcept
{
auto now = LLDate::now().secondsSinceEpoch();
// count calls to pick_next()
++mSwitches;
// pick_next() is called when the previous fiber has suspended, and we
// need to pick another. Did the previous pick_next() call pick the main
// fiber? (Or is this the first pick_next() call?) If so, it's the main
// fiber that just suspended.
if ((! mPrevCtx) || mPrevCtx->get_id() == mMainID)
{
mMainLast = now;
}
else
{
// How long did we spend in the fiber that just suspended?
// Don't bother with long runs of the main fiber, since (a) it happens
// pretty often and (b) it's moderately likely that we've reached here
// from the canonical yield at the top of mainloop, and what we'd want
// to know about is whatever the main fiber was doing in the
// *previous* iteration of mainloop.
F64 elapsed{ now - mResumeTime };
LLCoros::CoroData& data{ LLCoros::get_CoroData(mPrevCtx->get_id()) };
// Find iterator to the first mHistogram key greater than elapsed.
auto past = data.mHistogram.upper_bound(elapsed);
// If the smallest key (mHistogram.begin()->first) is greater than
// elapsed, then we need not bother with this timeslice.
if (past != data.mHistogram.begin())
{
// Here elapsed was greater than at least one key. Back off to the
// previous entry and increment that count. If it's end(), backing
// off gets us the last entry -- assuming mHistogram isn't empty.
llassert(! data.mHistogram.empty());
++(--past)->second;
LL::WorkQueue::ptr_t queue{ getWorkQueue() };
// make sure the queue exists
if (queue)
{
// If it proves difficult to track down *why* the fiber spent so
// much time, consider also binding and reporting
// boost::stacktrace::stacktrace().
queue->post(
[name=data.getName(), elapsed]
{
LL_WARNS_ONCE("LLCoros.scheduler")
<< "Coroutine " << name << " ran for "
<< elapsed << " seconds" << LL_ENDL;
});
}
}
}
boost::fibers::context* next;
// When the main fiber is ready, and it's been more than mTimeslice since
// the main fiber last ran, it's time to intervene.
F64 elapsed(now - mMainLast);
if (mMainCtx && elapsed > mTimeslice)
{
// We claim that the main fiber is not only stored in mMainCtx, but is
// also queued (somewhere) in our ready list.
llassert(mMainCtx->ready_is_linked());
// The usefulness of a doubly-linked list is that, given only a
// pointer to an item, we can unlink it.
mMainCtx->ready_unlink();
// Instead of delegating to round_robin::pick_next() to pop the head
// of the queue, override by returning mMainCtx.
next = mMainCtx;
/*------------------------- logging stuff --------------------------*/
// Unless this log tag is enabled, don't even bother posting.
LL_DEBUGS("LLCoros.scheduler") << " ";
// This feature is inherently hard to verify. The logging in the
// lambda below seems useful, but also seems like a lot of overhead
// for a coroutine context switch. Try posting the logging lambda to a
// ThreadPool to offload that overhead. However, if this is still
// taking an unreasonable amount of context-switch time, this whole
// passage could be skipped.
// Record this event for logging, but push it off to a thread pool to
// perform that work.
LL::WorkQueue::ptr_t queue{ getWorkQueue() };
// The work queue we're looking for might not exist right now.
if (queue)
{
// Bind values. Do NOT bind 'this' to avoid cross-thread access!
// It would be interesting to know from what queue position we
// unlinked the main fiber, out of how many in the ready list.
// Unfortunately round_robin::rqueue_ is private, not protected,
// so we have no access.
queue->post(
[switches=mSwitches, start=mStart, elapsed, now]
{
U32 runtime(U32(now) - U32(start));
U32 minutes(runtime / 60u);
U32 seconds(runtime % 60u);
// use stringize to avoid lasting side effects to the
// logging ostream
LL_DEBUGS("LLCoros.scheduler")
<< "At time "
<< stringize(minutes, ":", std::setw(2), std::setfill('0'), seconds)
<< " (" << switches << " switches), coroutines took "
<< stringize(std::setprecision(4), elapsed)
<< " sec, main coroutine jumped queue"
<< LL_ENDL;
});
}
LL_ENDL;
/*----------------------- end logging stuff ------------------------*/
}
else
{
// Either the main fiber isn't yet ready, or it hasn't yet been
// mTimeslice seconds since the last time the main fiber ran. Business
// as usual.
next = super::pick_next();
}
// super::pick_next() could also have returned the main fiber, which is
// why this is a separate test instead of being folded into the override
// case above.
if (next && next->get_id() == mMainID)
{
// we're about to resume the main fiber: it's no longer "ready"
mMainCtx = nullptr;
}
mPrevCtx = next;
// remember when we resumed this fiber so our next call can measure how
// long the previous resumption was
mResumeTime = LLDate::now().secondsSinceEpoch();
return next;
}
LL::WorkQueue::ptr_t scheduler::getWorkQueue()
{
// Cache a weak_ptr to our target work queue, presuming that
// std::weak_ptr::lock() is cheaper than WorkQueue::getInstance().
LL::WorkQueue::ptr_t queue{ mQueue.lock() };
// We probably started before the relevant WorkQueue was created.
if (! queue)
{
// Try again to locate the specified WorkQueue.
queue = LL::WorkQueue::getInstance(qname);
mQueue = queue;
}
return queue;
}
void scheduler::use()
{
boost::fibers::use_scheduling_algorithm<scheduler>();
}
} // namespace llcoro
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