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Date: Fri, 19 Jul 2002 19:04:52 -0400
From: Johannes Erdfelt <johannes@erdfelt.com>
To: Austin Gonyou <austin@digitalroadkill.net>
Cc: David Rees <dbr@greenhydrant.com>, linux-kernel@vger.kernel.org
Subject: Re: 2.4.19rc2aa1 VM too aggressive?
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On Fri, Jul 19, 2002, Austin Gonyou <austin@digitalroadkill.net> wrote:
> On Fri, 2002-07-19 at 16:03, Johannes Erdfelt wrote:
> > Web server. The only writing is for the log files, which is relatively
> > minimal.
> 
> But IMHO, you are using prefork, and not a threaded model correct?

Yes, it's a prefork.

> > One thing also, is there is lots of process creation in this example.
> > For a variety of reasons, PHP programs are forked often from the Apache
> > server.
> 
> Also, here, even as a DSO, which I think you may not be running PHP as,
> (cgi vs. dso), you will use a bit of memory, on top of apache, every
> time the new child is created by apache to handle incoming requests.

Use both, but for legacy reasons there's still a signficant amount of
children being forked for the CGI like version (caused by SSI).

The memory size for these children is about 40MB (which is strange in
itself), and a couple per second get executed. However, they are very
quick and typically won't see any in ps, but occassionally 1 or 2 will
be seen.

> > The systems running an older kernel (like RedHat's 2.4.9-21) are much
> > more consistent in their usage of memory. There are no 150MB swings in
> > cache utiliziation, etc.
> 
> Hrrmmm....I'd suggest a 2.4.17 or 2.4.19-rc1-aa2 in that case. I promise
> you'll see drastic improvements over that kernel.

2.4.17 wasn't good last time I tried it, but I've have much better results
from Andrea's patches. I'll create 2.4.19-rc1-aa2 kernel and see how
that fares.

> > What's really odd in the vmstat output is the fact that there is no disk
> > I/O that follows these wild swings. Where is this cache memory coming
> > from? Or is the accounting just wrong?
> 
> I think the accounting is quite correct. Let's look real quick. 

I suspect it's correct as well, but that doesn't mean something else
isn't wrong :)

> <vmstat>
> > > >    procs                      memory    swap          io     system  cpu
> > > >  3  0  0 106036 502288  10812  67236   0   0     0     0  802   494  46  37  17
> > > >  5  0  2 106032 476188  10844  91496   0   0     4   316  905   573  54  37   8
> > > > 16  0  2 106032 355400  10844 203880   0   0     4     0  909   540  51  49   0
> > > > 10  0  2 106024 340108  10852 221548   0   0    28     0  975   659  36  64   0
> > > >  0  0  0 106024 528340  10852  43572   0   0     4     0  569   426  17  17  67
> > > >  0  1  0 106024 531304  10852  43612   0   0     4     0  542   342   9  14 
> </vmstat>
> 
> Now let's take a closer look....
> 
> <vmstat2>
> > > > 16  0  2 106032 355400  10844 203880   0   0     4     0  909   540  51  49   0
> > > > 10  0  2 106024 340108  10852 221548   0   0    28     0  975   659  36  64   0
> </vmstat2>
> 
> Notice you're memory utilization jumps here as your free is given to
> cache.

Are you saying that the cache value is the amount of memory available to
be used by the cache, or actually used by the cache?

It was my understanding that it's the memory actually used by the cache.
If that's the case, I don't understand where the data to fill the cache
is coming from with these blips.

> <vmstat3>
> > > >  0  0  0 106024 528340  10852  43572   0   0     4     0  569   426  17  17  67
> > > >  0  1  0 106024 531304  10852  43612   0   0     4     0  542   342   9  14 
> </vmstat3>
> 
> And then back again, probably on process termination.

There are couple per second of those processes, so I would expect this
to happen all of the time or atleast much more often.

> At that rate, it's all in-memory shuffling going on, and for preforks,
> that very likely is the case.

One thing to note as well is a significant amount of system time spent
during these situations as well. It looks like a lot of time is spent
managing something.

It's obvious the workload is inefficient, but it's constantly
inefficient which is why these blips are strange.

JE

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