Re: What is the impact of setting ServerLimit higher than MaxClients? (Apache Users)

On Tue, Aug 20, 2013 at 5:30 PM, Rose, John B <jbrose@xxxxxxx> wrote:

Can you expand on this a bit?

"But you could measure actual shm use with diff values."

ServerLimit controls the size of the scoreboard.

The scoreboard is shared memory (the "shm" abbreviation above).

Measure shared memory use by httpd using some tool. I used System Monitor on recent Ubuntu. For a 64-bit build of httpd 2.4 with the worker MPM with (exaggerated since it needs for multiple threads per process), changing ServerLimit from 5000 to 20000 increased shared memory use of the parent httpd from about 80MB to 306 MB.

I'd guess you're using a lot less than 80MB with ServerLimit 2000 and prefork.

Thanks

From: Jeff Trawick <trawick@xxxxxxxxx>
Reply-To: "users@xxxxxxxxxxxxxxxx" <users@xxxxxxxxxxxxxxxx>

Date: Tuesday, August 20, 2013 5:13 PM

To: "users@xxxxxxxxxxxxxxxx" <users@xxxxxxxxxxxxxxxx>
Subject: Re: What is the impact of setting ServerLimit higher than MaxClients?

On Aug 20, 2013 5:05 PM, "Rose, John B" <jbrose@xxxxxxx> wrote:
>
> Thanks for replying.
>
> Yes, it is prefork MPM.
>
> Reading this in the Apache documentation …
>
> "Do not set the value of this directive any higher than what you might want to set MaxClients to."
>
> It also says …
>
> "Special care must be taken when using this directive. If ServerLimit is set to a value much higher than necessary, extra, unused shared memory will be allocated"
>
> What is classified as "much higher?"
>
> Is there a negative (or positive) impact of any kind of, for example, setting ServerLimit to 2000 and MaxClients to 512?

Unclear what much higher means, especially on a modern machine. I'd be extremely surprised if 2000 has any noticeable impact. But you could measure actual shm use with diff values.
>
> http://httpd.apache.org/docs/2.2/mod/mpm_common.html#serverlimit
>
> Thanks
>
>
> From: Jeff Trawick <trawick@xxxxxxxxx>
> Reply-To: "users@xxxxxxxxxxxxxxxx" <users@xxxxxxxxxxxxxxxx>
> Date: Tuesday, August 20, 2013 4:50 PM
> To: "users@xxxxxxxxxxxxxxxx" <users@xxxxxxxxxxxxxxxx>
> Subject: Re: What is the impact of setting ServerLimit higher than MaxClients?
>
> On Aug 20, 2013 4:26 PM, "Rose, John B" <jbrose@xxxxxxx> wrote:
> >
> > What is the impact of setting the ServerLimit to a value significantly more than your MaxClients value? Say with a MaxClients set to 512 and ServerLimit of 2000?
> >
>
> Assuming prefork MPM:
>
> You can increase maxclients across graceful restart, up to serverlimit
>
> Threaded MPMs: comparing maxclients vs server limit doesn't work without also accounting for threadsperchild
>
> >
> >

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