update compat

[lttv.git] / ltt / branches / poly / doc / developer / lttng-atomic-up.txt

diff --git a/ltt/branches/poly/doc/developer/lttng-atomic-up.txt b/ltt/branches/poly/doc/developer/lttng-atomic-up.txt
index bbbc61f6b5d91b1e1f2856a9fe8cf17f92fb53b0..9ce3482cc4517ebfbca6d926775584e0cf660694 100644 (file)
--- a/ltt/branches/poly/doc/developer/lttng-atomic-up.txt
+++ b/ltt/branches/poly/doc/developer/lttng-atomic-up.txt
@@ -73,4 +73,59 @@ Res : 767.51 cycles per loop
 
 (205.63-255.83)/255.83 * 100% = 19.62 %
 
+
+Difference between
+cmpxchg 2967855/20000 = 148.39 cycles or 49.46 ns
+cmpxchg-up 540577/20000 = 27.02 cycles or 9.00 ns
+irq save/restore 12636562/20000 = 631.82 cycles 210.60 ns
+
+
+
+* Memory ordering
+
+offset
+written by local CPU
+read by local CPU and other CPUs (reader)
+
+commit count
+written by local CPU
+read by local CPU and other CPUs (reader)
+
+consumed
+written by any CPU
+read by any CPU
+
+data
+written by local CPU
+read by any CPU
+
+
+test done in the reader :
+if ( consumed < offset )
+  if ( subbuf.commit_count == multiple of SUBBUFSIZE)
+    read data
+    inc consumed
+
+
+We must guarantee the following ordering :
+* offset
+Seen from the local CPU :
+offset must always be incremented before the data is written (already
+consistent)
+
+Seen from other cpus :
+offset and data can be written out of order
+(because offset is always incremented : in an out of order case, offset is lower
+than the actual data ready, but the commit_count _has_ to be incremented to read
+the data (and is preceded by a store fence)
+
+* commit_count
+commit_count must always be seen by other CPUs after the data has been written.
+Therefore, we must put a store fence before the commit_count write. (smp_wmb)
+
+* consumed
+Rarely updated, use LOCK prefix. Acts as a full memory barrier.
+
+
+
 Mathieu Desnoyers, November 2006