[THIN] Re: Context Switches
- From: "Schneider, Chad M." <CMSchneider@xxxxxxxxx>
- To: "'thin@xxxxxxxxxxxxx'" <thin@xxxxxxxxxxxxx>
- Date: Thu, 27 Mar 2003 10:05:56 -0600
It was more of a thinking out loud question.
Thanks for the insight.
:)
-----Original Message-----
From: Braebaum, Neil [mailto:Neil.Braebaum@xxxxxxxxxxxxxxxxx]
Sent: Thursday, March 27, 2003 10:04 AM
To: 'thin@xxxxxxxxxxxxx'
Subject: [THIN] Re: Context Switches
I was sorta expecting you to join up the dots a bit, there, Chad ;-)
You do know what a page fault is?
If not - it's what occurs when a paging operation (read: request for a page
of memory) is not in memory - so it has to go elsewhere to get it.
As to the why's - well it's your system! ;-)
Any number of reasons - but as a starter, are you running any memory
optimising products?
Neil
> -----Original Message-----
> From: Schneider, Chad M. [mailto:CMSchneider@xxxxxxxxx]
> Sent: 27 March 2003 15:56
> To: 'thin@xxxxxxxxxxxxx'
> Subject: [THIN] Re: Context Switches
>
> Yup, now why so many page faults?
>
> -----Original Message-----
> From: Braebaum, Neil [mailto:Neil.Braebaum@xxxxxxxxxxxxxxxxx]
> Sent: Thursday, March 27, 2003 9:55 AM
> To: 'thin@xxxxxxxxxxxxx'
> Subject: [THIN] Re: Context Switches
>
> From this, you could reasonably infer the reason behind your
> context switches...?
>
> > -----Original Message-----
> > From: Schneider, Chad M. [mailto:CMSchneider@xxxxxxxxx]
> > Sent: 27 March 2003 15:51
> > To: 'thin@xxxxxxxxxxxxx'
> > Subject: [THIN] Re: Context Switches
> >
> > In response to this, my page faults spike in accordance with
> > the context switches. Same time frames.
> >
> > -----Original Message-----
> > From: Bernd Harzog [mailto:Bernd.Harzog@xxxxxxxxxxx]
> > Sent: Thursday, March 13, 2003 4:19 AM
> > To: thin@xxxxxxxxxxxxx
> > Subject: [THIN] Re: Context Switches
> >
> > Neil (and Chad),
> >
> > I spoke to Kevin Goodman about this (the person who founded
> > our company and who wrote TScale). He said that there is some
> > debate about whether the switch from user to kernel mode
> > within a process triggers a context switch - so Neil, you are
> > probably right. He did offer one insight as to what might be
> > happening to cause the number to go through the roof. He said
> > that it is likely that two or more processes are competing
> > for a locked resource, and the CPU is simply continuously
> > switching between them to see which one gets the locked
> > resource first. His suggestion as to how to track this down
> > is to do the following. Go into PerfMon and look at the
> > Process Performance Object. This object allows you to track
> > counters on a per process basis (per instance of a program
> > running on the server). Unfortunately, you cannot track
> > context switches this way, but you can track threads. So, his
> > suggestion is to select Threads as the object, and then to
> > select all of the instances of the main applications that are
> > running (you are going to end up with 25 sets of thread data
> > if you have 25 users of an application).
> >
> > The behavior that you ought to see is that when the context
> > switch problem occurs, a set of processes will drive the
> > thread count counters for themselves through the roof. Then
> > you will know which set of processes is causing the problem.
> > Once you know that then you are going to have to backtrack
> > through the configuration of those applications on your
> > servers (and whatever else supports the client running on the
> > terminal servers), and possibly also backtrack into the
> > source code (if it is your application).
> >
> > -----Original Message-----
> > From: Braebaum, Neil
> > [mailto:Neil.Braebaum@xxxxxxxxxxxxxxxxx]=20
> > Sent: Thursday, March 13, 2003 4:35 AM
> > To: 'thin@xxxxxxxxxxxxx'
> > Subject: [THIN] Re: Context Switches
> >
> > Bernd - the "two kinds of context switches" - really equates to one.
> >
> > It really is just the OS switching from a user mode to kernel
> > mode for a process. When this occurs between two (pseudo)
> > concurrent sessions, you have to realise a certain degree of
> > this is inevitable.
> >
> > There is no discernable different in "types" of context
> > switch, though. Merely the cause.
> >
> > If the OS switches / timeslices between user mode processes -
> > you have to realise and accept that in doing so, some kernel
> > process activity will occur. And to be fair, some user mode
> > activity may well produce or spawn some kernel mode activity.
> >
> > There is no true distinction in the type of context switch -
> > so it's a bit unfair to say that MS don't differentiate or
> > provide a mechanism to measure this - there is no true
> > difference. What matters is the cause.
> >
> > > -----Original Message-----
> > > From: Bernd Harzog [mailto:Bernd.Harzog@xxxxxxxxxxx]=20
> > > Sent: 12 March 2003 18:04
> > > To: thin@xxxxxxxxxxxxx
> > > Subject: [THIN] Re: Context Switches
> > >=20
> > > Let me add my two cents. There are as Ron said two kinds of=20
> > >context switches. One kind occurs when the processor switches=20
> > >between two user mode tasks. So, if Ron and I are both=20
> > running an
> > >application on the same server when the CPU=20 switches
> > between doing
> > >work for Ron and work for me, that=20 causes a context
> > switch. That is
> > >a GOOD context switch,=20 because it is indicative of the
> CPU doing
> > >what it is suppose=20 to do, which is move so rapidly
> between doing
> > >work for=20 different users that each user never notices.
> > The second
> > >type=20 of context switch occurs when the CPU goes from user mode
> > >to=20 kernel mode, and then back again (two Context Switches).=20
> > > These are BAD context switches, because when the CPU goes=20
> > > into Kernel mode it blocks user mode operations and slows=20
> > > everything down.
> > >=20
> > > So, you need to find out what kind of context switches you=20
> > > are getting. Since MS does not do us the favor of breaking=20
> > > them out, the only way to find out is to look at other=20
> > > indicators of heavy Kernel Mode operations and see if they=20
> > > are pegged as well. Try looking at pages/sec and page=20
> > > faults/sec. If you see a heavy correlation between these and=20
> > > the context switches you may have an indicator that it is=20
> > > kernel mode related. If it is, call me, because I have=20
> > > something new that I cannot talk about on the list that we=20
> > > might want to discuss.
> > >=20
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