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Re: [Xen-devel] [PATCH 2/2] xenoprofile: Add IBS support

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 2/2] xenoprofile: Add IBS support
From: Wei Wang2 <wei.wang2@xxxxxxx>
Date: Tue, 10 Aug 2010 14:44:48 +0200
Cc: robert.richter@xxxxxxx, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On Monday 09 August 2010 13:41:11 Jan Beulich wrote:
> >>> On 30.07.10 at 15:28, Wei Wang2 <wei.wang2@xxxxxxx> wrote:
> >
> >--- a/drivers/oprofile/oprofile_files.c
> >+++ b/drivers/oprofile/oprofile_files.c
> >@@ -21,7 +21,7 @@
> > #include "oprof.h"
> >
> > unsigned long fs_buffer_size = 131072;
> >-unsigned long fs_cpu_buffer_size = 8192;
> >+unsigned long fs_cpu_buffer_size = 131072;
> Why is this needed at all, why this much of an increase, and why not
> conditional upon CONFIG_XEN?
> Jan

Hi Jan
I had observed over 50% samples lose using following configuration:
opcontrol --start --event=IBS_OP_ALL:50000 --no-vmlinux
if fs_cpu_buffer_size = 8192. The same issue was also seen on native 2.6.34 
kernel. In my case, extending fs_cpu_buffer_size or increasing sampling 
period can fix this issue.

Since each IBS_OP sample will occupy 14 entries in cpu buffer compared with 
only 1 entry for event-based sample, I suspect that the default size of 
per-cpu buffer would not be enough for IBS_OP mode in minimum sampling period 
(50000). I simply extended fs_cpu_buffer_size just to make sure it works for 
current userland tools. If we intent to force a longer minimum sampling 
period for IBS_OP_ALL in future release of tools, we could keep 
fs_cpu_buffer_size = 8192.


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