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Active Reparenting

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In Young Brothers Wait Concept (YBWC) available slaves are
booked by the split point master, then start to search below
the assigned split point and, once finished, return in idle
state waiting to be booked by another master.

This patch introduces "Active Reparenting" so that when a
slave finishes its job on the assigned split point, instead
of passively waiting to be booked, searches a suitable active
split point and reprents itselfs to that split point. Then
immediately starts to search below the split point in exactly
the same way of the others split point's slaves. This reduces
to zero the time waiting in idle loop and should increase
scalability especially whit many (8 or more) cores.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
This commit is contained in:
Marco Costalba
2012-04-10 18:00:31 +01:00
parent d66b765eb6
commit 44432f67d7
4 changed files with 49 additions and 1 deletions
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43
src/search.cpp
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@@ -1859,6 +1859,49 @@ void Thread::idle_loop(SplitPoint* sp_master) {
// our feet by the sp master. Also accessing other Thread objects is
// unsafe because if we are exiting there is a chance are already freed.
lock_release(sp->lock);
// Try to reparent to another split point. Only for slave threads
// that are not master of any active split point.
if ( !sp_master
&& !is_searching
&& !do_sleep
&& !do_exit
&& !splitPointsCnt
&& Threads.size() > 2)
{
for (int i = 0; i < Threads.size(); i++)
{
SplitPoint* oldest = &Threads[i].splitPoints[0];
// Find the first oldest split point with still all slaves running
if ( Threads[i].splitPointsCnt
&& oldest->slavesMask == oldest->allSlavesMask
&& !single_bit(oldest->allSlavesMask))
{
lock_grab(oldest->lock);
lock_grab(Threads.splitLock); // Needed by is_searching
// Retest all under lock protection, we are in the middle
// of a race storm !
if ( !is_searching
&& !do_sleep
&& !do_exit
&& Threads[i].splitPointsCnt
&& oldest->slavesMask == oldest->allSlavesMask
&& !single_bit(oldest->allSlavesMask))
{
oldest->slavesMask |= 1ULL << idx; // allSlavesMask is not updated
curSplitPoint = oldest;
is_searching = true;
}
lock_release(Threads.splitLock);
lock_release(oldest->lock);
break; // Exit anyhow, only one try (enough in 99% of cases)
}
}
}
}
}
}