Re: [Xen-devel] Re: how to avoid lost trace records?

[George Dunlap <dunlapg@xxxxxxxxx>]

Olaf,

Dang, 8 megs per cpu -- but I guess that's really not so much overhead
on a big machine; and it's definitely worth getting around the lost
records issue.  Send the T_INFO_PAGES patch to the list, and see what
Keir thinks.

There's probably a way to modify xenalyze to do start up gzip
directly; may not be a bad idea.

 -George

On Sat, Nov 20, 2010 at 8:21 PM, Olaf Hering <olaf@xxxxxxxxx> wrote:
> On Fri, Nov 19, Olaf Hering wrote:
>
>>
>> Today I inspected the xenalyze and the dump-raw output and noticed that
>> huge number of lost trace records, even when booted with tbuf_size=200:
>>
>> grep -wn 1f001 log.sles11_6.xentrace.txt.dump-raw
>> 274438:R p 5 o000000000063ffd4    1f001 4 t0000006d215b3c6b [ b6aed 57fff 
>> 9e668fb6 51 ]
> ...
>> That means more than 740K lost entries on cpu5,3,2,1,0.
>> Is this expected?
>
> After reading the sources more carefully, its clear now.
> There are a few constraints:
>
> If booted with tbuf_size=N, tracing starts right away and fills up the
> buffer until xentrace collects its content. So entries will be lost.
>
> Once I just ran xentrace -e all > output, which filled up the whole disk
> during my testing. So I changed the way to collect the output to a
> compressed file:
>
>  # mknod pipe p
>  # gzip -v9 < pipe > output.gz &
>  # xentrace -e all pipe &
>
> This means xentrace will stall until gzip has made room in the pipe.
> Which also means xentrace cant collect more data from the tracebuffer
> while waiting. So that is the reason for the lost entries.
>
> Now I changed T_INFO_PAGES in trace.c from 2 to 16, and reduced the
> compression rate to speedup gzip emptying the pipe.
>
>  # mknod pipe p
>  # nice -n -19 gzip -v1 < pipe > output.gz &
>  # nice -n -19 xentrace -s 1 -S 2031 -e $(( 0x10f000 )) pipe &
>
>
> This means no more lost entries even with more than one guest running.
>
>
> Olaf
>
>
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