kernel: Approaching the limit on PV entries...

[Bob Johnson]

On 10/10/08, Mark Tinguely <tinguely at casselton.net> wrote:
>>  > vm.pmap.pv_entry_count: 583006
>>  > vm.pmap.shpgperproc: 200
>>  > vm.pmap.pv_entry_max: 2243305
>>  >
>>  > The system:
>>  > FreeBSD .... 7.0-RELEASE-p5 FreeBSD 7.0-RELEASE-p5 #0: Wed Oct  1
>>  > 07:51:58 UTC 2008
>>  > root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  amd64
>>  >
>>  > Can someone briefly explain what this is telling me and how to decide
>>  > which sysctl to increase? I have found some old postings that predate
>>  > the sysctls that suggested increasing shpgperproc in the kernel
>>  > configuration, about 50 at a time until the problem goes away, but I
>>  > still have no clue what that is accomplishing.
>
> what (simplified):
> the pv_entry helps the virtual memory system track physical pages, so a
> physical page can be shared with another process or another virtual address.
>
> In the i386/amd64 the pv_entry entries are allocated in page size "chunks"
> on a per process memory map basis. This helps reduce redundant pointers
> and overall saves memory.
>
> "shpgperproc" can be read as "the number of shared pages per proceess".
> pv_entry_max is calculated from shpgperproc (and on the amd64, shpgperproc
> can be derived from setting the vm.pmap.pv_entry_max).
>
> On the amd64, the values can be adjusted by sysctl, but on the i386
> the values must be compiled into the kernel.
>
> There are some automatic adjustments in the calculation of the number
> of pv_entry, but the warnings are given early enough to help aid in the
> tweaking of the value. The advice of slowly increasing vm.pmap.shpgperproc
> is probably the best solution. I would adjust up slower than 50 (25%
> increase seems to be pretty high).
>
> --Mark Tinguely.
>

Thanks. I'll see what happens.

In amd64/7.0 is there any chance running out of pv_entrys would show
up as failures in interprocess communication rather than a panic? The
original symptom was that certain web pages (or jailed servers, I'm
not sure) were unreachable, as if the firewall were misconfigured,
until the system was rebooted. I didn't get to look at the system
before it was rebooted, and I find very little in the logs to explain
what was going on.

Thanks again,

- Bob
  fbsdlists at gmail.com