Re: [PATCH] increase ple_gap default to 64

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



Riel,
Thanks for your patch. I have changed the ple_gap to 128 on xen side, but forget the patch for KVM:(

A little bit big is no harm, but more perf data is better.



Rik van Riel wrote:
On some CPUs, a ple_gap of 41 is simply insufficient to ever trigger
PLE exits, even with the minimalistic PLE test from kvm-unit-tests.

http://git.kernel.org/?p=virt/kvm/kvm-unit-tests.git;a=commitdiff;h=eda71b28fa122203e316483b35f37aaacd42f545

For example, the Xeon X5670 CPU needs a ple_gap of at least 48 in
order to get pause loop exits:

# modprobe kvm_intel ple_gap=47
# taskset 1 /usr/local/bin/qemu-system-x86_64 -device testdev,chardev=log -chardev stdio,id=log -kernel x86/vmexit.flat -append ple-round-robin -smp 2
VNC server running on `::1:5900'
enabling apic
enabling apic
ple-round-robin 58298446
# rmmod kvm_intel
# modprobe kvm_intel ple_gap=48
# taskset 1 /usr/local/bin/qemu-system-x86_64 -device testdev,chardev=log -chardev stdio,id=log -kernel x86/vmexit.flat -append ple-round-robin -smp 2
VNC server running on `::1:5900'
enabling apic
enabling apic
ple-round-robin 36616

Increase the ple_gap to 64 to be on the safe side. Is this enough for a CPU with HT that has a busy sibling thread, or should it be even larger? On the X5670, loading up the sibling thread
with an infinite loop does not seem to increase the required ple_gap.

Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 81fcbe9..0e38b8e 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -90,14 +90,14 @@ module_param(vmm_exclusive, bool, S_IRUGO);
  * These 2 parameters are used to config the controls for Pause-Loop Exiting:
  * ple_gap:    upper bound on the amount of time between two successive
  *             executions of PAUSE in a loop. Also indicate if ple enabled.
- *             According to test, this time is usually small than 41 cycles.
+ *             According to test, this time is usually smaller than 64 cycles.
  * ple_window: upper bound on the amount of time a guest is allowed to execute
  *             in a PAUSE loop. Tests indicate that most spinlocks are held for
  *             less than 2^12 cycles
  * Time is measured based on a counter that runs at the same rate as the TSC,
  * refer SDM volume 3b section 21.6.13 & 22.1.3.
  */
-#define KVM_VMX_DEFAULT_PLE_GAP    41
+#define KVM_VMX_DEFAULT_PLE_GAP    64
 #define KVM_VMX_DEFAULT_PLE_WINDOW 4096
 static int ple_gap = KVM_VMX_DEFAULT_PLE_GAP;
 module_param(ple_gap, int, S_IRUGO);

--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html


[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]
  Powered by Linux