[PATCH] libports: reduce thread starvation on message floods

[Richard Braun]

libports/manage-multithread.c: Add #include <mach/thread_switch.h>
(ports_manage_port_operations_multithread): Make threads depress
their priority on startup.
---
 libports/manage-multithread.c |   11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/libports/manage-multithread.c b/libports/manage-multithread.c
index 82fa2ac..2c690d2 100644
--- a/libports/manage-multithread.c
+++ b/libports/manage-multithread.c
@@ -23,6 +23,7 @@
 #include <assert.h>
 #include <cthreads.h>
 #include <mach/message.h>
+#include <mach/thread_switch.h>
 
 void
 ports_manage_port_operations_multithread (struct port_bucket *bucket,
@@ -123,6 +124,16 @@ ports_manage_port_operations_multithread (struct 
port_bucket *bucket,
       int timeout;
       error_t err;
 
+      /* XXX To reduce starvation, the priority of new threads is initially
+        depressed. This helps already existing threads complete their job
+        and be recycled to handle new messages. The duration of this
+        depression is made a function of the total number of threads because
+        more threads implies more contention, and the priority of threads
+        blocking on a contented spin lock is also implicitely depressed.
+        The lock isn't needed, since an approximation is sufficient. */
+      timeout = (((totalthreads - 1) / 100) + 1) * 10;
+      thread_switch(MACH_PORT_NULL, SWITCH_OPTION_DEPRESS, timeout);
+
       if (hook)
        (*hook) ();
 
-- 
1.7.10.4