[vbox-dev] [PATCH] FreeBSD: fix issue with under-tick waits

[Bernhard Fröhlich]

Another patch for FreeBSD kernel module from Andriy. License is MIT as usual.


---------- Forwarded message ----------
From: Andriy Gapon <avg at freebsd.org>
Date: Thu, Dec 13, 2012 at 2:05 PM
Subject: issue with under-tick waits
To: vbox at freebsd.org, freebsd-emulation at freebsd.org



I think that the following patch is needed in VirtualBox 4.2.4 kernel module:

--- sleepqueue-r0drv-freebsd.h.orig     2012-12-12 20:09:29.675203802 +0200
+++ sleepqueue-r0drv-freebsd.h  2012-12-12 20:11:53.604203131 +0200
@@ -82,6 +82,8 @@
     uint64_t cTicks = ASMMultU64ByU32DivByU32(uTimeout, hz,
UINT32_C(1000000000));
     if (cTicks >= INT_MAX)
         return RTSEMWAIT_FLAGS_INDEFINITE;
+    else if (cTicks == 0 && uTimeout > 0)
+        pWait->iTimeout     = 1;
     else
         pWait->iTimeout     = (int)cTicks;
 #endif

Without this patch any waits for periods shorter than a single tick return
immediately leading to a lot of unnecessary spinning.  For example, I
observe that
my guest's idle loop does a lot of sleeps with periods slightly
shorter than 1 ms
(1/hz), e.g. 900us.  All that waiting turns into pure spinning and
VirtualBox eats
100% of a core.
The above patch improves the situation significantly.  Also, it (approximately)
follows what tvtohz does.

--
Andriy Gapon