From ecd057376de847d40e1e29e2bf33e95eae3f835f Mon Sep 17 00:00:00 2001 From: Martin Quinson Date: Tue, 9 Aug 2022 01:17:39 +0200 Subject: [PATCH] Don't test statequality: it's ~15h w/o DPOR and hard to optimize --- examples/cpp/CMakeLists.txt | 16 +++++---- .../s4u-mc-failing-assert-statequality.tesh | 2 +- src/mc/compare.cpp | 34 ++++++++++++++++++- 3 files changed, 44 insertions(+), 8 deletions(-) diff --git a/examples/cpp/CMakeLists.txt b/examples/cpp/CMakeLists.txt index 4e13fab9ee..1e4132106e 100644 --- a/examples/cpp/CMakeLists.txt +++ b/examples/cpp/CMakeLists.txt @@ -216,12 +216,16 @@ endforeach() # Test non-DPOR reductions on a given MC test foreach(example mc-failing-assert) if(SIMGRID_HAVE_MC) - ADD_TESH(s4u-${example}-statequality --setenv bindir=${CMAKE_CURRENT_BINARY_DIR}/${example} - --setenv libdir=${CMAKE_BINARY_DIR}/lib - --setenv platfdir=${CMAKE_HOME_DIRECTORY}/examples/platforms - --setenv srcdir=${CMAKE_CURRENT_SOURCE_DIR}/${example} - --cd ${CMAKE_CURRENT_SOURCE_DIR}/${example} - ${CMAKE_HOME_DIRECTORY}/examples/cpp/${example}/s4u-${example}-statequality.tesh) +# State equality is not tested because it would take about 15 hours to run that test on my machine. +# We should first optimize mmalloc_heap_differ() which takes ~4sec for each pair to compare (maybe {175 x 174/ 2} pairs here) +# See the comment on mmalloc_heap_differ() in compare.cpp for more info on why it's hard to optimize. +# +# ADD_TESH(s4u-${example}-statequality --setenv bindir=${CMAKE_CURRENT_BINARY_DIR}/${example} +# --setenv libdir=${CMAKE_BINARY_DIR}/lib +# --setenv platfdir=${CMAKE_HOME_DIRECTORY}/examples/platforms +# --setenv srcdir=${CMAKE_CURRENT_SOURCE_DIR}/${example} +# --cd ${CMAKE_CURRENT_SOURCE_DIR}/${example} +# ${CMAKE_HOME_DIRECTORY}/examples/cpp/${example}/s4u-${example}-statequality.tesh) ADD_TESH(s4u-${example}-nodpor --setenv bindir=${CMAKE_CURRENT_BINARY_DIR}/${example} --setenv libdir=${CMAKE_BINARY_DIR}/lib diff --git a/examples/cpp/mc-failing-assert/s4u-mc-failing-assert-statequality.tesh b/examples/cpp/mc-failing-assert/s4u-mc-failing-assert-statequality.tesh index 5489a1e182..97c8085422 100644 --- a/examples/cpp/mc-failing-assert/s4u-mc-failing-assert-statequality.tesh +++ b/examples/cpp/mc-failing-assert/s4u-mc-failing-assert-statequality.tesh @@ -2,7 +2,7 @@ ! expect return 1 ! timeout 300 -$ ${bindir:=.}/../../../bin/simgrid-mc --cfg=model-check/visited:20 -- ${bindir:=.}/s4u-mc-failing-assert ${platfdir}/small_platform.xml --log=root.thresh:critical +$ ${bindir:=.}/../../../bin/simgrid-mc --cfg=model-check/visited:10000 -- ${bindir:=.}/s4u-mc-failing-assert ${platfdir}/small_platform.xml --log=root.thresh:critical > [0.000000] [xbt_cfg/INFO] Configuration change: Set 'model-check/visited' to '20' > [0.000000] [mc_dfs/INFO] Start a DFS exploration. Reduction is: dpor. > [0.000000] [mc_ModelChecker/INFO] ************************** diff --git a/src/mc/compare.cpp b/src/mc/compare.cpp index 8acb0dd135..849059b214 100644 --- a/src/mc/compare.cpp +++ b/src/mc/compare.cpp @@ -201,6 +201,37 @@ static bool heap_area_differ(const RemoteProcess& process, StateComparator& stat const Snapshot& snapshot1, const Snapshot& snapshot2, HeapLocationPairs* previous, Type* type, int pointer_level); +/* Compares the content of each heap fragment between the two states, at the bit level. + * + * This operation is costly (about 5 seconds per snapshots' pair to compare on a small program), + * but hard to optimize because our algorithm is too hackish. + * + * Going at bit level can trigger syntaxtic differences on states that are semantically equivalent. + * + * Padding bytes constitute the first source of such syntaxtic difference: Any malloced memory contains spaces that + * are not used to enforce the memory alignment constraints of the CPU. So, cruft of irrelevant changes could get + * added on these bits. But this case is handled properly, as any memory block is zeroed by mmalloc before being handled + * back, not only for calloc but also for malloc. So the memory interstices due to padding bytes are properly zeroed. + * + * Another source of such change comes from the order of mallocs, that may well change from one execution path to + * another. This will change the malloc fragment in which the data is stored and the pointer values (syntaxtic + * difference) while the semantic of the state remains the same. + * + * To fix this, this code relies on a hugly hack. When we see a difference during the bit-level comparison, + * we first check if it could be explained by a pointer-to-block difference. Ie, if when interpreting the memory + * area containing that difference as a pointer, I get the pointer to a valid fragment in the heap (in both snapshots). + * + * This is why we cannot pre-compute a bit-level hash of the heap content: we discover the pointers to other memory + * fragment when a difference is found during the bit-level exploration. Fixing this would require to save typing + * information about the memory fragments, which is something that could be done with https://github.com/tudasc/TypeART + * This would give us all pointers in the mallocated memory, allowing the graph traversal needed to precompute the hash. + * + * Using a hash without paying attention to malloc fragment reordering would lead to false negatives: + * semantically equivalent states would be detected as [syntaxically] different. It's of no importance for the + * state-equality reduction (we would re-explore semantically equivalent states), but it would endanger the soundness + * of the liveness model-checker, as state-equality is used to detect the loops that constitute the accepting states of + * the verified property. So we could miss counter-examples to the verified property. Not good. Not good at all. + */ static bool mmalloc_heap_differ(const RemoteProcess& process, StateComparator& state, const Snapshot& snapshot1, const Snapshot& snapshot2) { @@ -1247,9 +1278,10 @@ bool Snapshot::operator==(const Snapshot& other) } } + XBT_VERB(" Compare heap..."); /* Compare heap */ if (mmalloc_heap_differ(process, state_comparator, *this, other)) { - XBT_VERB("(%ld - %ld) Different heap (mmalloc_compare)", this->num_state_, other.num_state_); + XBT_VERB("(%ld - %ld) Different heap (mmalloc_heap_differ)", this->num_state_, other.num_state_); return false; } -- 2.20.1