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Q142497: SNA Server Processes Show Huge Virtual Byte Size in PerfMon

Article: Q142497
Product(s): Microsoft SNA Server
Version(s): WINDOWS:2.0,2.1,2.11,3.0
Operating System(s): 
Keyword(s): kbnetwork
Last Modified: 13-JUN-2001

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The information in this article applies to:

- Microsoft SNA Server, versions 2.0, 2.1, 2.11, 3.0, on platform(s):
   - the operating system: Microsoft Windows NT 
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SUMMARY
=======

When viewing the virtual byte size of any SNA Server service or Win32
application which uses the SNA Server API's, Windows NT Performance Monitor
shows the Win32 process' virtual byte size to be larger than 154MB.

SNA Server reserves a 154MB contiguous region in the process' virtual address
space, though only 64KB of physical memory is initially allocated or committed.
Additional physical memory is allocated only when needed. Also, this memory is
shared between all SNA Server processes running on the computer, and only one
copy is allocated even if there are several SNA applications running.

Since this large virtual memory area is not actually allocated out of available
physical memory, other processes running on the computer are not be affected,
nor does it affect the Windows NT page file. The committed memory can be found
by viewing the "working set" size of each process.

MORE INFORMATION
================

SNA Server processes running locally on a Windows NT computer communicate with
each other using a shared memory region. Each SNA Server process (that is,
SnaBase, SNA Server, link services, and applications which use the SNA API's)
communicate with other SNA Server processes on the computer by writing to
buffers in this memory region which is then available for other SNA Server
processes to access.

When SnaBase is started, it reserves this shared memory region as a Windows NT
virtual memory mapped file. The size of this file is determined by the size of
all possible SNA Server internal communication buffers which can potentially be
allocated (on SNA Server 2.11, this is 154,300,416 bytes). However, out of this
large potential size, the initial allocation size is less than 64KB (16KB for
process headers, 32KB for elements and 8KB for headers).

SNA Server logs event 684 and 685 to indicate changes in the buffer pool memory
allocation. Event 684 is logged when over half of the maximum number of buffer
pools has been allocated. Event 685 is logged if Windows NT has returned an
error to our VirtualAlloc call, or if the maximum number of buffer pool
extensions have been allocated. Full internal tracing on the component logging
these events indicates the cause of the 685 error. The text for these events are
included below:

  Event 684
  A buffer pool audit has been triggered by a pool extension.

  EXPLANATION

  This audit contains information about the size of the buffer pool,
  how many headers and elements each process is using, etc.

                  Pool      Free     Alloced      Max

  Trusted Hdrs
  Trusted Elts
  Non-Trusted Hdrs
  Non-Trusted Elts

  Comname    Pid    Trusted   Hdrs   Elts

  SnaBase
  SnaServr
  SnaDlc1
  ...

  ACTION

  None, unless there is some other problem. In that case, report
  this log along with any others.

  Event 685

  An attempt was made to extend a buffer pool, but the related pool
  had reached its maximum size. The affected component is terminating,
  and an audit of the buffer pools just before termination is attached

  EXPLANATION
  This error is almost always due to either an SNA component (including
  possibly an emulator) losing buffers or a slow emulator (or RUI
  application) using a session without pacing.

  ACTION
  Contact your support personnel.

  Excessive logging of this event may indicate that an SNA Server process
  running on the machine is not releasing buffers properly, which could
  indicate a potential memory leak.


Additional query words: prodsna

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Keywords          : kbnetwork 
Technology        : kbAudDeveloper kbSNAServSearch
Version           : WINDOWS:2.0,2.1,2.11,3.0

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