svn commit: r47019 - head/en_US.ISO8859-1/htdocs/news/status

Sun Jul 19 19:59:02 UTC 2015

Author: emaste (src committer)
Date: Sun Jul 19 19:59:00 2015
New Revision: 47019
URL: https://svnweb.freebsd.org/changeset/doc/47019

Log:
  Update PCID report based on some feedback from wblock
  
  Differential Revision:	https://reviews.freebsd.org/D3127

Modified:
  head/en_US.ISO8859-1/htdocs/news/status/report-2015-04-2015-06.xml

Modified: head/en_US.ISO8859-1/htdocs/news/status/report-2015-04-2015-06.xml
==============================================================================

--- head/en_US.ISO8859-1/htdocs/news/status/report-2015-04-2015-06.xml	Sun Jul 19 19:27:26 2015	(r47018)
+++ head/en_US.ISO8859-1/htdocs/news/status/report-2015-04-2015-06.xml	Sun Jul 19 19:59:00 2015	(r47019)
@@ -1091,19 +1091,20 @@
     </links>
 
     <body>
-      <p>Process-Context Identifiers (PCIDs) is a feature of the
+      <p>A Process-Context Identifier (PCID) is a performance enhancing
+	feature of the
 	Translation Lookaside Buffer (TLB) on Intel processors,
 	introduced with the Sandy Bridge micro-architecture.  It
 	allows the TLB to
 	simultaneously cache translation information for several
 	address spaces, and gives an opportunity for the operating
-	system context switch code to avoid flushing the TLB on the
+	system context switch code to avoid flushing the TLB upon
 	process switch.  Each cached translation is tagged with some
 	context identifier, and at context switch time, the operating
 	system instructs the processor which context is becoming
 	active.  The feature slightly reduces context switch time by
-	avoiding flush, and more importantly, it reduces the warm-up
-	period for the thread after a context switch.</p>
+	avoiding TLB flush, and more importantly, reduces the warm-up
+	period for a thread after context switch.</p>
 
       <p>&os; already used PCID, but the existing implementation
 	had several shortcomings.  The <tt>amd64</tt> pmap (the
@@ -1113,9 +1114,9 @@
 	on the context switch.  The bitmap was used to direct
 	Inter-Processor Interrupts to the marked CPU when the
 	operating system needed to perform TLB invalidation.  The most
-	important deficiency of the implementation is the increase of
-	TLB invalidation IPIs since the bitmap could only grow until
-	full TLB shootdown is performed.  It increases the TLB rate,
+	important deficiency of the implementation was the increase of
+	TLB invalidation IPIs, since the bitmap could only grow until
+	full TLB shootdown was performed.  It increased the TLB rate,
 	which negated the positive effects of avoiding TLB flushes on
 	large machines.  Secondarily, the bitmap maintenance in both
 	the pmap and the context code was quite complicated, leading
@@ -1125,13 +1126,14 @@
       <p>The new PCID implementation uses an algorithm described in
 	the U. Vahalia book "UNIX Internals: The New Frontiers".  The
 	algorithm is already used, for example, by the MIPS pmap for
-	assigning the ASIDs to software-managed TLB entries.  The pmap
+	assigning Address Space Identifiers (ASIDs) to software-managed
+        TLB entries.  The pmap
 	maintains a per-CPU generation count, which is assigned to the
 	next unused PCID when the context is activated on CPU.  TLB
 	invalidation includes resetting the generation count, which
-	causes reallocation of PCID when a context switch is
+	causes reallocation of the PCID when a context switch is
 	performed.  As result, the new implementation issues exactly
-	the same amount of shootdown IPIs as pmap which does not
+	the same amount of shootdown IPIs as a pmap which does not
 	utilize PCID.</p>
 
       <p>Another change included with the PCID rewrite is a move of
@@ -1139,9 +1141,8 @@
 	making the algorithm easier to understand and validate.</p>
 
       <p>Measurements done with <tt>hwpmc(4)</tt> on a Haswell machine
-	indicated that the new implementation reduced the amount of
-	data TLB misses up to 10 times, without an impact on the IPI
-	counters.</p>
+	indicated that the new implementation reduced the TLB miss rate by
+	up to 10 times, without an increase in TLB shootdown IPIs.</p>
 
       <p>The rewrite was committed to HEAD at r282684.</p>
 
