From 8e116e357872634b0126957add47423b96dba014 Mon Sep 17 00:00:00 2001
From: Jason Volk <jason@zemos.net>
Date: Wed, 18 Sep 2019 20:29:18 -0700
Subject: [PATCH] ircd::db::prefetcher: Add yield point.

---
 ircd/db.cc | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/ircd/db.cc b/ircd/db.cc
index b16fa3cf2..02edfb7c2 100644
--- a/ircd/db.cc
+++ b/ircd/db.cc
@@ -3866,7 +3866,6 @@ ircd::db::prefetcher::operator()(column &c,
 		return false;
 	}
 
-	const ctx::critical_assertion ca;
 	queue.emplace_back(d, c, key);
 	queue.back().snd = now<steady_point>();
 	++request_counter;
@@ -3879,9 +3878,19 @@ ircd::db::prefetcher::operator()(column &c,
 	if(db::request.wouldblock())
 	{
 		dock.notify_one();
+
+		// If the user sets NO_BLOCKING we honor their request to not
+		// context switch for a prefetch. However by default we want to
+		// control queue growth, so we insert voluntary yield here to allow
+		// prefetch operations to at least be processed before returning to
+		// the user submitting more prefetches.
+		if(likely(!test(opts, db::get::NO_BLOCKING)))
+			ctx::yield();
+
 		return true;
 	}
 
+	const ctx::critical_assertion ca;
 	++directs_counter;
 	this->handle();
 	return true;