Replace interleave_size with channel_hash

This patch replaces the previously introduced interleave_size memory
channel decoding mechanism with a more powerful channel_hash. Decoding
can now be based upon an arbitrary mask of address bits, which will be
XORed together to determine the target channel. Note that this drops
support for more than two channels, but TripleChannel controllers will
probably use much more complicated decoding mechanisms anyway.

It also includes the findmask program, which offers a crude method to
guess the decoding mask from an unknown memory controller for enterprising
users. Use at your own risk.

Signed-off-by: Julius Werner <jwerner@chromium.org>

8 files changed, 238 insertions, 63 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index e044974..2179b42 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -1,4 +1,5 @@
 bin_PROGRAMS = stressapptest
+noinst_PROGRAMS = findmask
 
 AM_DEFAULT_SOURCE_EXT=.cc
 
@@ -29,3 +30,4 @@ HFILES += adler32memcpy.h
 HFILES += logger.h
 
 stressapptest_SOURCES = $(MAINFILES) $(CFILES) $(HFILES)
+findmask_SOURCES = findmask.c findmask.inc
diff --git a/src/Makefile.in b/src/Makefile.in
index f62d1ac..65470cb 100644
--- a/src/Makefile.in
+++ b/src/Makefile.in
@@ -36,6 +36,7 @@ build_triplet = @build@
 host_triplet = @host@
 target_triplet = @target@
 bin_PROGRAMS = stressapptest$(EXEEXT)
+noinst_PROGRAMS = findmask$(EXEEXT)
 subdir = src
 DIST_COMMON = $(srcdir)/Makefile.am $(srcdir)/Makefile.in \
 	$(srcdir)/stressapptest_config.h.in
@@ -48,7 +49,10 @@ CONFIG_HEADER = stressapptest_config.h
 CONFIG_CLEAN_FILES =
 CONFIG_CLEAN_VPATH_FILES =
 am__installdirs = "$(DESTDIR)$(bindir)"
-PROGRAMS = $(bin_PROGRAMS)
+PROGRAMS = $(bin_PROGRAMS) $(noinst_PROGRAMS)
+am_findmask_OBJECTS = findmask.$(OBJEXT)
+findmask_OBJECTS = $(am_findmask_OBJECTS)
+findmask_LDADD = $(LDADD)
 am__objects_1 = main.$(OBJEXT)
 am__objects_2 = os.$(OBJEXT) os_factory.$(OBJEXT) pattern.$(OBJEXT) \
 	queue.$(OBJEXT) sat.$(OBJEXT) sat_factory.$(OBJEXT) \
@@ -63,17 +67,17 @@ DEFAULT_INCLUDES = -I.@am__isrc@
 depcomp = $(SHELL) $(top_srcdir)/depcomp
 am__depfiles_maybe = depfiles
 am__mv = mv -f
+COMPILE = $(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) \
+	$(CPPFLAGS) $(AM_CFLAGS) $(CFLAGS)
+CCLD = $(CC)
+LINK = $(CCLD) $(AM_CFLAGS) $(CFLAGS) $(AM_LDFLAGS) $(LDFLAGS) -o $@
 CXXCOMPILE = $(CXX) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) \
 	$(AM_CPPFLAGS) $(CPPFLAGS) $(AM_CXXFLAGS) $(CXXFLAGS)
 CXXLD = $(CXX)
 CXXLINK = $(CXXLD) $(AM_CXXFLAGS) $(CXXFLAGS) $(AM_LDFLAGS) $(LDFLAGS) \
 	-o $@
-COMPILE = $(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) \
-	$(CPPFLAGS) $(AM_CFLAGS) $(CFLAGS)
-CCLD = $(CC)
-LINK = $(CCLD) $(AM_CFLAGS) $(CFLAGS) $(AM_LDFLAGS) $(LDFLAGS) -o $@
-SOURCES = $(stressapptest_SOURCES)
-DIST_SOURCES = $(stressapptest_SOURCES)
+SOURCES = $(findmask_SOURCES) $(stressapptest_SOURCES)
+DIST_SOURCES = $(findmask_SOURCES) $(stressapptest_SOURCES)
 ETAGS = etags
 CTAGS = ctags
 DISTFILES = $(DIST_COMMON) $(DIST_SOURCES) $(TEXINFOS) $(EXTRA_DIST)
@@ -188,11 +192,12 @@ HFILES = os.h pattern.h queue.h sat.h worker.h sattypes.h \
 	finelock_queue.h error_diag.h disk_blocks.h adler32memcpy.h \
 	logger.h
 stressapptest_SOURCES = $(MAINFILES) $(CFILES) $(HFILES)
+findmask_SOURCES = findmask.c findmask.inc
 all: stressapptest_config.h
 	$(MAKE) $(AM_MAKEFLAGS) all-am
 
 .SUFFIXES:
-.SUFFIXES: .cc .o .obj
+.SUFFIXES: .c .cc .o .obj
 $(srcdir)/Makefile.in:  $(srcdir)/Makefile.am  $(am__configure_deps)
 	@for dep in $?; do \
 	  case '$(am__configure_deps)' in \
@@ -277,6 +282,12 @@ uninstall-binPROGRAMS:
 
 clean-binPROGRAMS:
 	-test -z "$(bin_PROGRAMS)" || rm -f $(bin_PROGRAMS)
+
+clean-noinstPROGRAMS:
+	-test -z "$(noinst_PROGRAMS)" || rm -f $(noinst_PROGRAMS)
+findmask$(EXEEXT): $(findmask_OBJECTS) $(findmask_DEPENDENCIES) 
+	@rm -f findmask$(EXEEXT)
+	$(LINK) $(findmask_OBJECTS) $(findmask_LDADD) $(LIBS)
 stressapptest$(EXEEXT): $(stressapptest_OBJECTS) $(stressapptest_DEPENDENCIES) 
 	@rm -f stressapptest$(EXEEXT)
 	$(CXXLINK) $(stressapptest_OBJECTS) $(stressapptest_LDADD) $(LIBS)
@@ -290,6 +301,7 @@ distclean-compile:
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/adler32memcpy.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/disk_blocks.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/error_diag.Po@am__quote@
+@AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/findmask.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/finelock_queue.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/logger.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/main.Po@am__quote@
@@ -301,6 +313,20 @@ distclean-compile:
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/sat_factory.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/worker.Po@am__quote@
 
+.c.o:
+@am__fastdepCC_TRUE@	$(COMPILE) -MT $@ -MD -MP -MF $(DEPDIR)/$*.Tpo -c -o $@ $<
+@am__fastdepCC_TRUE@	$(am__mv) $(DEPDIR)/$*.Tpo $(DEPDIR)/$*.Po
+@AMDEP_TRUE@@am__fastdepCC_FALSE@	source='$<' object='$@' libtool=no @AMDEPBACKSLASH@
+@AMDEP_TRUE@@am__fastdepCC_FALSE@	DEPDIR=$(DEPDIR) $(CCDEPMODE) $(depcomp) @AMDEPBACKSLASH@
+@am__fastdepCC_FALSE@	$(COMPILE) -c $<
+
+.c.obj:
+@am__fastdepCC_TRUE@	$(COMPILE) -MT $@ -MD -MP -MF $(DEPDIR)/$*.Tpo -c -o $@ `$(CYGPATH_W) '$<'`
+@am__fastdepCC_TRUE@	$(am__mv) $(DEPDIR)/$*.Tpo $(DEPDIR)/$*.Po
+@AMDEP_TRUE@@am__fastdepCC_FALSE@	source='$<' object='$@' libtool=no @AMDEPBACKSLASH@
+@AMDEP_TRUE@@am__fastdepCC_FALSE@	DEPDIR=$(DEPDIR) $(CCDEPMODE) $(depcomp) @AMDEPBACKSLASH@
+@am__fastdepCC_FALSE@	$(COMPILE) -c `$(CYGPATH_W) '$<'`
+
 .cc.o:
 @am__fastdepCXX_TRUE@	$(CXXCOMPILE) -MT $@ -MD -MP -MF $(DEPDIR)/$*.Tpo -c -o $@ $<
 @am__fastdepCXX_TRUE@	$(am__mv) $(DEPDIR)/$*.Tpo $(DEPDIR)/$*.Po
@@ -431,7 +457,8 @@ maintainer-clean-generic:
 	@echo "it deletes files that may require special tools to rebuild."
 clean: clean-am
 
-clean-am: clean-binPROGRAMS clean-generic mostlyclean-am
+clean-am: clean-binPROGRAMS clean-generic clean-noinstPROGRAMS \
+	mostlyclean-am
 
 distclean: distclean-am
 	-rm -rf ./$(DEPDIR)
@@ -501,17 +528,18 @@ uninstall-am: uninstall-binPROGRAMS
 .MAKE: all install-am install-strip
 
 .PHONY: CTAGS GTAGS all all-am check check-am clean clean-binPROGRAMS \
-	clean-generic ctags distclean distclean-compile \
-	distclean-generic distclean-hdr distclean-tags distdir dvi \
-	dvi-am html html-am info info-am install install-am \
-	install-binPROGRAMS install-data install-data-am install-dvi \
-	install-dvi-am install-exec install-exec-am install-html \
-	install-html-am install-info install-info-am install-man \
-	install-pdf install-pdf-am install-ps install-ps-am \
-	install-strip installcheck installcheck-am installdirs \
-	maintainer-clean maintainer-clean-generic mostlyclean \
-	mostlyclean-compile mostlyclean-generic pdf pdf-am ps ps-am \
-	tags uninstall uninstall-am uninstall-binPROGRAMS
+	clean-generic clean-noinstPROGRAMS ctags distclean \
+	distclean-compile distclean-generic distclean-hdr \
+	distclean-tags distdir dvi dvi-am html html-am info info-am \
+	install install-am install-binPROGRAMS install-data \
+	install-data-am install-dvi install-dvi-am install-exec \
+	install-exec-am install-html install-html-am install-info \
+	install-info-am install-man install-pdf install-pdf-am \
+	install-ps install-ps-am install-strip installcheck \
+	installcheck-am installdirs maintainer-clean \
+	maintainer-clean-generic mostlyclean mostlyclean-compile \
+	mostlyclean-generic pdf pdf-am ps ps-am tags uninstall \
+	uninstall-am uninstall-binPROGRAMS
 
 
 # Tell versions [3.59,3.63) of GNU make to not export all variables.
diff --git a/src/findmask.c b/src/findmask.c
new file mode 100644
index 0000000..d8ec300
--- /dev/null
+++ b/src/findmask.c
@@ -0,0 +1,138 @@
+/* Copyright 2013 Google Inc. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+
+/*
+ * This "tool" can be used to brute force the XOR bitmask that a memory
+ * controller uses to interleave addresses onto its two channels. To use it,
+ * you need to have a bunch of addresses that are known to go to only one
+ * of the memory channels... easiest way to get these is to run stressapptest on
+ * a machine while holding a soldering iron close to the chips of one channel.
+ * Generate about a thousand failures and extract their physical addresses
+ * from the output. Write them to findmask.inc in a way that forms a valid
+ * definition for the addrs array. Make and run on a big machine.
+ *
+ * The program iterates over all possible bitmasks within the first NUM_BITS,
+ * parallelizing execution over NUM_THREADS. Every integer is masked
+ * onto all supplied addresses, counting the amount of times this results in
+ * an odd or even amount of bits. If all but NOISE addresses fall on one side,
+ * it will print that mask to stdout. Note that the script will always "find"
+ * the mask 0x0, and may also report masks such as 0x100000000 depending on
+ * your test machines memory size... you will need to use your own judgement to
+ * interpret the results.
+ *
+ * As the program might run for a long time, you can send SIGUSR1 to it to
+ * output the last mask that was processed and get a rough idea of the
+ * current progress.
+ */
+
+#include <pthread.h>
+#include <signal.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+#define NOISE 20
+#define NUM_BITS 32
+#define NUM_THREADS 128  // keep this a power of two
+
+static uint64_t addrs[] = {
+#include "findmask.inc"
+};
+static uint64_t lastmask;
+
+__attribute__((optimize(3, "unroll-loops")))
+void* thread_func(void* arg) {
+  register uint64_t mask;
+  register uintptr_t num = (uintptr_t)arg;
+
+  for (mask = num; mask < (1ULL << (NUM_BITS + 1)); mask += NUM_THREADS) {
+    register const uint64_t* cur;
+    register int a = 0;
+    register int b = 0;
+
+    for (cur = addrs; (char*)cur < (char*)addrs + sizeof(addrs); cur++) {
+#ifdef __x86_64__
+      register uint64_t addr asm("rdx") = *cur & mask;
+      register uint32_t tmp asm("ebx");
+
+      // Behold: the dark bit counting magic!
+      asm (
+        // Fold high and low 32 bits onto each other
+        "MOVl %%edx, %%ebx\n\t"
+        "SHRq $32, %%rdx\n\t"
+        "XORl %%ebx, %%edx\n\t"
+        // Fold high and low 16 bits onto each other
+        "MOVl %%edx, %%ebx\n\t"
+        "SHRl $16, %%edx\n\t"
+        "XORw %%bx, %%dx\n\t"
+        // Fold high and low 8 bits onto each other
+        "XORb %%dh, %%dl\n\t"
+        // Invoke ancient 8086 parity flag (only counts lowest byte)
+        "SETnp %%bl\n\t"
+        "SETp %%dl\n\t"
+        // Stupid SET instruction can only affect the lowest byte...
+        "ANDl $1, %%ebx\n\t"
+        "ANDl $1, %%edx\n\t"
+        // Increment either 'a' or 'b' without needing another branch
+        "ADDl %%ebx, %2\n\t"
+        "ADDl %%edx, %1\n\t"
+        : "=b" (tmp), "+r"(a), "+r"(b) : "d"(addr) : "cc");
+
+#else  // generic processor
+      register uint64_t addr = *cur & mask;
+      register uint32_t low = (uint32_t)addr;
+      register uint32_t high = (uint32_t)(addr >> 32);
+
+      // Takes about twice as long as the version above... take that GCC!
+      __builtin_parity(low) ^ __builtin_parity(high) ? a++ : b++;
+#endif
+
+      // Early abort: probably still the most valuable optimization in here
+      if (a >= NOISE && b >= NOISE) break;
+    }
+
+    if (a < NOISE) b = a;
+    if (b < NOISE) {
+      printf("Found mask with just %d deviations: 0x%llx\n", b, mask);
+      fflush(stdout);
+    }
+
+    // I'm a little paranoid about performance: don't write to memory too often
+    if (!(mask & 0x7ff)) lastmask = mask;
+  }
+
+  return 0;
+}
+
+void signal_handler(int signum) {
+  printf("Received signal... currently evaluating mask 0x%llx!\n", lastmask);
+  fflush(stdout);
+}
+
+int main(int argc, char** argv) {
+  uintptr_t i;
+  pthread_t threads[NUM_THREADS];
+
+  signal(SIGUSR1, signal_handler);
+
+  for (i = 0; i < NUM_THREADS; i++)
+    pthread_create(&threads[i], 0, thread_func, (void*)i);
+
+  for (i = 0; i < NUM_THREADS; i++)
+    pthread_join(threads[i], 0);
+
+  return 0;
+}
diff --git a/src/findmask.inc b/src/findmask.inc
new file mode 100644
index 0000000..e76f72f
--- /dev/null
+++ b/src/findmask.inc
@@ -0,0 +1,4 @@
+// This is the body of a uintptr_t array definition. Fill in your own addresses.
+0x116bb312c, // example values (can be >32 bit)
+0x38d3c5ad,  // replace with your own
+0x77c1e96d   // don't forget: no comma after the last one
diff --git a/src/os.cc b/src/os.cc
index b73b18c..7cae23b 100644
--- a/src/os.cc
+++ b/src/os.cc
@@ -261,21 +261,22 @@ bool OsLayer::AdlerMemcpyWarm(uint64 *dstmem, uint64 *srcmem,
 }
 
 
-// Translate physical address to memory module name.
-// Assumes simple round-robin interleaving between memory channels of
-// 'interleave_size_' sized chunks, with repeated 'channel_width_'
+// Translate physical address to memory module/chip name.
+// Assumes interleaving between two memory channels based on the XOR of
+// all address bits in the 'channel_hash' mask, with repeated 'channel_width_'
 // blocks with bits distributed from each chip in that channel.
 int OsLayer::FindDimm(uint64 addr, char *buf, int len) {
   static const string unknown = "DIMM Unknown";
-  if (!modules_) {
+  if (!channels_) {
     snprintf(buf, len, "%s", unknown.c_str());
     return 0;
   }
 
-  // Find channel by counting interleave units (typically cachelines),
-  // and mod by number of channels.
-  vector<string>& channel = (*modules_)[
-      (addr / interleave_size_) % modules_->size()];
+  // Find channel by XORing address bits in channel_hash mask.
+  uint32 low = (uint32)(addr & channel_hash_);
+  uint32 high = (uint32)((addr & channel_hash_) >> 32);
+  vector<string>& channel = (*channels_)[
+      __builtin_parity(high) ^ __builtin_parity(low)];
 
   // Find dram chip by finding which byte within the channel
   // by address mod channel width, then divide the channel
diff --git a/src/os.h b/src/os.h
index 86e0c4e..a928577 100644
--- a/src/os.h
+++ b/src/os.h
@@ -58,11 +58,11 @@ class OsLayer {
   }
 
   // Set parameters needed to translate physical address to memory module.
-  void SetDramMappingParams(int interleave_size, int channel_width,
-                            vector< vector<string> > *modules) {
-    interleave_size_ = interleave_size;
+  void SetDramMappingParams(uintptr_t channel_hash, int channel_width,
+                            vector< vector<string> > *channels) {
+    channel_hash_ = channel_hash;
     channel_width_ = channel_width;
-    modules_ = modules;
+    channels_ = channels;
   }
 
   // Initializes data strctures and open files.
@@ -269,8 +269,8 @@ class OsLayer {
   bool  use_posix_shm_;          // Use 4k page shmem?
   bool  dynamic_mapped_shmem_;   // Conserve virtual address space.
   int   shmid_;                  // Handle to shmem
-  vector< vector<string> > *modules_;  // Memory module names per channel.
-  int interleave_size_;          // Channel interleaving chunk size.
+  vector< vector<string> > *channels_;  // Memory module names per channel.
+  uint64 channel_hash_;          // Mask of address bits XORed for channel.
   int channel_width_;            // Channel width in bits.
 
   int64 regionsize_;             // Size of memory "regions"
diff --git a/src/sat.cc b/src/sat.cc
index 0679ea1..4f4e684 100644
--- a/src/sat.cc
+++ b/src/sat.cc
@@ -572,12 +572,12 @@ bool Sat::Initialize() {
 
   if (min_hugepages_mbytes_ > 0)
     os_->SetMinimumHugepagesSize(min_hugepages_mbytes_ * kMegabyte);
-  if (modules_.size() > 0) {
+  if (channels_.size() > 0) {
     logprintf(6, "Log: Decoding memory: %dx%d bit channels,"
-        " %d byte burst size, %d modules per channel (x%d)\n",
-        modules_.size(), channel_width_, interleave_size_, modules_[0].size(),
-        channel_width_/modules_[0].size());
-    os_->SetDramMappingParams(interleave_size_, channel_width_, &modules_);
+        "%d modules per channel (x%d), decoding hash 0x%x\n",
+        channels_.size(), channel_width_, channels_[0].size(),
+        channel_width_/channels_[0].size(), channel_hash_);
+    os_->SetDramMappingParams(channel_hash_, channel_width_, &channels_);
   }
 
   if (!os_->Initialize()) {
@@ -650,7 +650,7 @@ Sat::Sat() {
   min_hugepages_mbytes_ = 0;
   freepages_ = 0;
   paddr_base_ = 0;
-  interleave_size_ = kCacheLineSize;
+  channel_hash_ = kCacheLineSize;
   channel_width_ = 64;
 
   user_break_ = false;
@@ -927,19 +927,19 @@ bool Sat::ParseArgs(int argc, char **argv) {
       continue;
     }
 
-    ARG_IVALUE("--interleave_size", interleave_size_);
+    ARG_IVALUE("--channel_hash", channel_hash_);
     ARG_IVALUE("--channel_width", channel_width_);
 
     if (!strcmp(argv[i], "--memory_channel")) {
       i++;
       if (i < argc) {
-        char *module = argv[i];
-        modules_.push_back(vector<string>());
-        while (char* next = strchr(module, ',')) {
-          modules_.back().push_back(string(module, next - module));
-          module = next + 1;
+        char *channel = argv[i];
+        channels_.push_back(vector<string>());
+        while (char* next = strchr(channel, ',')) {
+          channels_.back().push_back(string(channel, next - channel));
+          channel = next + 1;
         }
-        modules_.back().push_back(string(module));
+        channels_.back().push_back(string(channel));
       }
       continue;
     }
@@ -990,22 +990,25 @@ bool Sat::ParseArgs(int argc, char **argv) {
   }
 
   // Validate memory channel parameters if supplied
-  if (modules_.size()) {
-    if (interleave_size_ <= 0 ||
-        interleave_size_ & (interleave_size_ - 1)) {
+  if (channels_.size()) {
+    if (channels_.size() == 1) {
+      channel_hash_ = 0;
+      logprintf(7, "Log: "
+          "Only one memory channel...deactivating interleave decoding.\n");
+    } else if (channels_.size() > 2) {
       logprintf(6, "Process Error: "
-          "Interleave size %d is not a power of 2.\n", interleave_size_);
+          "Triple-channel mode not yet supported... sorry.\n");
       bad_status();
       return false;
     }
-    for (uint i = 0; i < modules_.size(); i++)
-      if (modules_[i].size() != modules_[0].size()) {
+    for (uint i = 0; i < channels_.size(); i++)
+      if (channels_[i].size() != channels_[0].size()) {
         logprintf(6, "Process Error: "
-            "Channels 0 and %d have a different amount of modules.\n",i);
+            "Channels 0 and %d have a different count of dram modules.\n",i);
         bad_status();
         return false;
       }
-    if (modules_[0].size() & (modules_[0].size() - 1)) {
+    if (channels_[0].size() & (channels_[0].size() - 1)) {
       logprintf(6, "Process Error: "
           "Amount of modules per memory channel is not a power of 2.\n");
       bad_status();
@@ -1018,9 +1021,9 @@ bool Sat::ParseArgs(int argc, char **argv) {
       bad_status();
       return false;
     }
-    if (channel_width_ / modules_[0].size() < 8) {
-      logprintf(6, "Process Error: "
-          "Chip width x%d must be x8 or greater.\n", channel_width_ / modules_[0].size());
+    if (channel_width_ / channels_[0].size() < 8) {
+      logprintf(6, "Process Error: Chip width x%d must be x8 or greater.\n",
+          channel_width_ / channels_[0].size());
       bad_status();
       return false;
     }
@@ -1095,8 +1098,8 @@ void Sat::PrintHelp() {
          "each CPU to be tested by that CPU\n"
          " --remote_numa    choose memory regions not associated with "
          "each CPU to be tested by that CPU\n"
-         " --interleave_size bytes  size in bytes of each channel's data as interleaved "
-         "between memory channels\n"
+         " --channel_hash   mask of address bits XORed to determine channel.\n"
+         "                  Mask 0x40 interleaves cachelines between channels\n"
          " --channel_width bits     width in bits of each memory channel\n"
          " --memory_channel u1,u2   defines a comma-separated list of names\n"
          "                          for dram packages in a memory channel.\n"
diff --git a/src/sat.h b/src/sat.h
index e867151..93d6b34 100644
--- a/src/sat.h
+++ b/src/sat.h
@@ -151,9 +151,8 @@ class Sat {
   int64 freepages_;                   // How many invalid pages we need.
   int disk_pages_;                    // Number of pages per temp file.
   uint64 paddr_base_;                 // Physical address base.
-  vector< vector<string> > modules_;  // Memory module names per channel.
-  int interleave_size_;               // Channel interleaving   chunk size in bytes.
-                                      // Usually cacheline sized.
+  vector< vector<string> > channels_; // Memory module names per channel.
+  uint64 channel_hash_;               // Mask of address bits XORed for channel.
   int channel_width_;                 // Channel width in bits.
 
   // Control flags.