David,
with my fix I was too quick.
d either has to be of type longlong_t (signed 64-bit integer) or of type
double. Then the comparison (d < 0) will also work.
Sorry for that.
Regards,
Michael
David Wong wrote:
Michael,
If d is an unsigned integer, won’t the test (d < 0) always be false?
My initial thought on the problem was to test if d is abnormally
large, which is what I would expect to happen when you subtract a
larger unsigned value from a smaller one.
Another option:
As long as the kernel is using 32 bit structures, we could convert the
unsigned integers to signed integers before doing the subtraction and
test for negative. But when the AIX kernel “upgrades” to 64 bit
structures, I imagine we would need to remove the “workaround” or we
risk the possibility of losing precision?
David Wong
Senior Systems Engineer
Management Dynamics, Inc.
Phone: 201-804-6127
[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
------------------------------------------------------------------------
*From:* Michael Perzl [mailto:[EMAIL PROTECTED]
*Sent:* Friday, March 30, 2007 12:25 PM
*To:* Andreas Schoenfeld
*Cc:* David Wong; [EMAIL PROTECTED];
ganglia-general@lists.sourceforge.net; [EMAIL PROTECTED]
*Subject:* Re: [Ganglia-general] Help! I have a petabyte/s network
(Martin Knoblauch)
Andreas,
thank you for taking the blame but you are off the hook here. ;-)
If I understood David correctly, he is using my AIX Ganglia RPM
packages with POWER5 extensions. Here most if not all implementation
of how the metrics are collected under AIX have been changed.
Everything is documented on my homepage
(http://www.perzl.org/ganglia/) though.
So everything what goes wrong here is entiremy my fault :-[
After some investigating and some discussions with Nigel I have come
to terms with the following facts regarding the bytes_in/bytes_out
problem:
- libperfstat (the library on AIX which obtains all the system
performance data) uses u_longlong_t data types (these are definitely
64-bit large).
- The AIX kernel internally, though, may probably not be using 64-bit
data types - more realistic is probably unsigned 32-bit - in order not
to break compatibility (my personal opinion)
- The consequence now is that integer overrun may occur much easier
with 32-bit data types than with 64-bit data types (we all probably
don't live long enough to see that happen).
Please take a look at my implementation of the bytes_in metric (the
bytes_out implementation is accordingly):
01 g_val_t
02 bytes_in_func( void )
03 {
04 g_val_t val;
05 perfstat_netinterface_total_t n;
06 static u_longlong_t last_bytes_in = 0, bytes_in;
07 static double last_time = 0.0;
08 double now, delta_t;
09 struct timeval timeValue;
10 struct timezone timeZone;
11
12 gettimeofday( &timeValue, &timeZone );
13
14 now = (double) (timeValue.tv_sec - boottime) + (timeValue.tv_usec /
1000000.0);
15
16 if (perfstat_netinterface_total( NULL, &n, sizeof(
perfstat_netinterface_total_t ), 1 ) == -1)
17 val.f = 0.0;
18 else
19 {
20 bytes_in = n.ibytes;
21
22 delta_t = now - last_time;
23
24 if ( delta_t )
25 val.f = (double) (bytes_in - last_bytes_in) / delta_t;
26 else
27 val.f = 0.0;
28
29 last_bytes_in = bytes_in;
30 }
31
32 last_time = now;
33
34 return( val );
35 }
In my opinion the overrun occurs in line #25 when "bytes_in <
last_bytes_in".
In my naivity I had assumed as both are of type u_longlong_t that an
integer overrun might never happen.
So to solve the overrun a check for "bytes_in < last_bytes_in" must be
introduced, something like:
u_longlong_t d;
d = bytes_in - last_bytes_in;
if (d < 0) d += ULONG_MAX;
and line #25 would essentially become
25 val.f = (double) d / delta_t;
Comments ?
Regards,
Michael
PS: David, the reason why you don't see it happen with pkts_in and
pkts_out is that probably no overrun so far has occurred but at some
point it will also happen.
PPS: David, if this is a solution (I want some comments on that
before, though) then I would be building new RPMs with the then
hopefully correct code.
Andreas Schoenfeld wrote:
Hi David and Martin,
I suppose the network code is still the code I wrote, so there are two
problems I know of:
1. yes there is a problem with owerflows
2. the shown network traffic is the sum of all network interfaces
including local loopback devices (lo0...).
Both Problems could lead to astonishing data transfer rate in ganglia.
Sorry I had promised to fix the problems, but there was to much other
work ...
Best regards
Andreas
Date: Thu, 29 Mar 2007 08:21:38 -0700 (PDT)
From: Martin Knoblauch <[EMAIL PROTECTED]> <mailto:[EMAIL PROTECTED]>
Subject: Re: [Ganglia-general] Help! I have a petabyte/s network
To: David Wong <[EMAIL PROTECTED]> <mailto:[EMAIL PROTECTED]>, [EMAIL PROTECTED]
<mailto:[EMAIL PROTECTED]>,
ganglia-general@lists.sourceforge.net
<mailto:ganglia-general@lists.sourceforge.net>
Message-ID: <[EMAIL PROTECTED]> <mailto:[EMAIL PROTECTED]>
Content-Type: text/plain; charset=iso-8859-1
David,
good catch. I will have to look at it for a bit.
Cheers
Martin
--- David Wong <[EMAIL PROTECTED]> <mailto:[EMAIL PROTECTED]> wrote:
I don't write much code nowadays, so I'm going to need a lot of help
with this.
I dug through the ganglia code and I found this interesting tidbit in
libmetrics/aix/metrics.c which may be indicative of the problem.
There's an assignment from cur_ninfo.ibytes to cur_net_stat.ibytes,
but
the types of the two variables are different.
net_stat::ibytes is a double:
struct net_stat{
double ipackets;
double opackets;
double ibytes;
double obytes;
} cur_net_stat;
and we have *ninfo declared here:
perfstat_netinterface_total_t ninfo[2],*last_ninfo, *cur_ninfo ;
libperfstat.h has perfstat_netinterface_total_t::ibytes as
u_longlong_t.
Does this code try to do what I think it is doing, i.e. assign an
unsigned 64 bit integer to a signed 64bit integer?
I'm willing to test the code if someone who's more adept at coding
and
building will take on the challenge.
It looks to me that the type mismatch will have to fixed in a few
places, such as CALC_NETSTAT, and we'll have to add an unsigned long
long to g_val_t too. Those are the ones I can see so far.
David Wong
Senior Systems Engineer
Management Dynamics, Inc.
Phone: 201-804-6127
[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
-----Original Message-----
From: Martin Knoblauch [mailto:[EMAIL PROTECTED]
Sent: Wednesday, March 28, 2007 12:00 PM
To: David Wong; ganglia-general@lists.sourceforge.net
<mailto:ganglia-general@lists.sourceforge.net>
Subject: Re: [Ganglia-general] Help! I have a petabyte/s network
David,
as far as I remember, the AIX metrics code had an
overflow/wrap-around
problem prior to 3.0.4. Maybe the fixes are not thorough enough.
The packets/sec are of course less affected.
Cheers
Martin
