[THIN] Re: latency

  • From: "Bernd Harzog" <bernd@xxxxxxxxxxx>
  • To: <thin@xxxxxxxxxxxxx>
  • Date: Fri, 4 Oct 2002 07:55:50 -0400

Neil,

Kevin Goodman (the founder of KevSoft, and the person who invented and wrote
TScale), believes that a substantial portion of the kernel mode activity
which occurs as the OS is using the CPU to keep its own house in order is
not reported in CPU utilization counters. A specific example would be if a
kernel mode driver were to go into an infinite loop, this would block user
mode activity and in turn cause CPU counters to go down to zero, while the
CPU would in fact be unavailable to do useful work for users.

We have observed the converse in many successful TScale installations.
Specifically, as TScale has eliminated page file activity (which is kernel
mode activity) from the execution threads of applications, CPU utilization
has gone up. The reason it has gone up is that now more CPU is available to
do useful work for users, and as the CPU does useful work for users the
system performs better, scales better, etc.

The bottom line answer to your question is that yes, we believe that CPU
utilization counters are misleading because they do not include CPU utilized
by the OS in kernel mode tasks. I will attempt to explain why this is the
case (because I do not know), but you could probably speculate and come to
the same analytical conclusion that we have.

Bernd Harzog
CEO
KevSoft Corporation
678-455-5509 x701
bernd@xxxxxxxxxxx


 -----Original Message-----
From:   thin-bounce@xxxxxxxxxxxxx [mailto:thin-bounce@xxxxxxxxxxxxx]  On
Behalf Of Braebaum, Neil
Sent:   Friday, October 04, 2002 4:37 AM
To:     'thin@xxxxxxxxxxxxx'
Subject:        [THIN] Re: latency


Bernd

Just to clarify - are you suggesting there are circumstances where counters
are not reporting, or not accounting for system / kernel activity (ie that
it is being underreported)?

Or merely that this information is somewhat obfuscated?

I find it difficult to believe that system / kernel activity is effectively
black-holing from a reporting / monitoring perspective - be it from task
manager, or perfmon.

I'd agree that system / kernel activity can be difficult to track down
exactly as to the cause, but I've *never* encountered scenarios where the
system has busy cpus, but misleading counters - except for when a machine is
so obviously *so* busy, that the monitoring application / utility isn't
getting a look in.

Neil

> -----Original Message-----
> From: Bernd Harzog [mailto:bernd@xxxxxxxxxxx]
> Sent: 03 October 2002 22:32
> To: thin@xxxxxxxxxxxxx
> Subject: [THIN] Re: latency
>
> The behavior that Eric describes is very interesting. The
> important point is a drop in CPU utilization when the latency
> occurs. What this means is two things; 1) The CPU utilization
> counter is not measuring whatever is causing the latency, 2)
> Something is causing the CPU to spend a lot of time doing
> something instead of useful work on behalf of user applications.
>
> There is a scenario which we have encountered at customers
> which fits into the above explanation. That scenario is that
> the time that the CPU spends on certain system level
> operations (kernel mode operations like Page File Writes, and
> Page Faults) is not counted by the CPU utilization counter,
> AND that something is making the CPU spend a lot of time
> these Kernel Mode operations. The things that can cause these
> kinds of Kernel Mode operations to block useful work include
> a saturated disk driver, a large number of queued up page
> writes, a large number of page faults and numerous other things.
>
> If you are experiencing this kind of behavior, and it is
> being caused by excessive paging (watch page file writes in
> Perfmon), then TSCale will help. If you are saturating your
> RAID drivers TScale cannot help.

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