[master] 6e47cd6ab Yield to newly bred threads

[Dridi Boukelmoune]

commit 6e47cd6abfa624552f72c7888ba9810b34a142f6
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Mon Oct 28 17:06:35 2019 +0100

    Yield to newly bred threads
    
    When we breed threads without a delay, we should make an effort to let
    them run as soon as possible and not have the cpu occupied with the
    herder code.
    
    This will increase the overhead due to additional context switching for
    the benefit of reducing latencies until new threads get to run.
    
    Effect seen with this vtc by @Dridi:
    
        varnishtest "over-breeding"
    
        varnish v1 -arg "-p thread_pools=1 -p thread_pool_min=10"
        varnish v1 -vcl {
                backend be none;
        } -start
    
        varnish v1 -expect MAIN.threads == 10
    
    Tested with the following script:
    
        ./varnishtest -i -k -j40 -n1000 tests/as_above.vtc |
        awk '/^#/ { print $5; }'|
        sort |
        uniq -c
    
    Multiple runs show a consistent drop of failed tests:
    
        before | after
        -------+------
            76 |    18
            67 |    24
            66 |    18

diff --git a/bin/varnishd/cache/cache_wrk.c b/bin/varnishd/cache/cache_wrk.c
index a0662c221..01a379ddd 100644
--- a/bin/varnishd/cache/cache_wrk.c
+++ b/bin/varnishd/cache/cache_wrk.c
@@ -53,6 +53,7 @@
 #include "config.h"
 
 #include <stdlib.h>
+#include <sched.h>
 
 #include "cache_varnishd.h"
 #include "cache_pool.h"
@@ -478,7 +479,10 @@ pool_breed(struct pool *qp)
 		VSC_C_main->threads++;
 		VSC_C_main->threads_created++;
 		Lck_Unlock(&pool_mtx);
-		VTIM_sleep(cache_param->wthread_add_delay);
+		if (cache_param->wthread_add_delay > 0.0)
+			VTIM_sleep(cache_param->wthread_add_delay);
+		else
+			sched_yield();
 	}
 
 	AZ(pthread_attr_destroy(&tp_attr));