svn commit: r187703 - head/share/man/man7

Tom Rhodes trhodes at
Sun Jan 25 18:15:23 PST 2009

Author: trhodes
Date: Mon Jan 26 02:15:22 2009
New Revision: 187703

  Better description of kern.ipc.maxpipekva.
  Discussed with:	kib


Modified: head/share/man/man7/tuning.7
--- head/share/man/man7/tuning.7	Mon Jan 26 01:08:24 2009	(r187702)
+++ head/share/man/man7/tuning.7	Mon Jan 26 02:15:22 2009	(r187703)
@@ -403,13 +403,16 @@ on the system.
 .Va kern.ipc.maxpipekva
-loader tunable is used to set a hard limit on the amount of pageable
-address space available.
-This value is auto tuned by the system, but may require manual tuning
-in a few rare cases.
+loader tunable is used to set a hard limit on the
+amount of kernel address space allocated to mapping of pipe buffers.
+Use of the mapping allows the kernel to eliminate a copy of the
+data from writer address space into the kernel, directly copying
+the content of mapped buffer to the reader.
 Increasing this value to a higher setting, such as `25165824' might
-improve performance on systems where pageable address space is
-quickly exhausted.
+improve performance on systems where space for mapping pipe buffers
+is quickly exhausted.
+This exhaustion is not fatal; however, and it will only cause pipes to
+to fall back to use double-copy.
 .Va kern.ipc.shm_use_phys

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