diff --git a/clients/drcachesim/tests/scheduler_launcher.cpp b/clients/drcachesim/tests/scheduler_launcher.cpp
index cedea03c401..bfebb7e5861 100644
--- a/clients/drcachesim/tests/scheduler_launcher.cpp
+++ b/clients/drcachesim/tests/scheduler_launcher.cpp
@@ -232,6 +232,22 @@ simulate_core(int ordinal, scheduler_t::stream_t *stream, const scheduler_t &sch
                 cur_segment_instrs = 0;
             }
         }
+#ifdef HAS_ZIP
+        else if (record.marker.type == dynamorio::drmemtrace::TRACE_TYPE_MARKER) {
+            if (record.marker.marker_type ==
+                dynamorio::drmemtrace::TRACE_MARKER_TYPE_CPU_ID) {
+                if (!op_cpu_schedule_file.get_value().empty()) {
+                    int cpu = (int)record.marker.marker_value;
+                    int output_cpuid = stream->get_output_cpuid();
+                    if (cpu != output_cpuid) {
+                        FATAL_ERROR("CPU marker %d on core #%d differs from output "
+                                    "stream CPU ID %d\n",
+                                    cpu, ordinal, output_cpuid);
+                    }
+                }
+            }
+        }
+#endif
     }
 }
 
diff --git a/clients/drcachesim/tools/invariant_checker.cpp b/clients/drcachesim/tools/invariant_checker.cpp
index d97cd15a079..90250def034 100644
--- a/clients/drcachesim/tools/invariant_checker.cpp
+++ b/clients/drcachesim/tools/invariant_checker.cpp
@@ -488,6 +488,11 @@ invariant_checker_t::parallel_shard_memref(void *shard_data, const memref_t &mem
             report_if_false(shard, shard->last_instr_count_marker_ == ASM_INSTR_COUNT,
                             "Incorrect instr count marker value");
         }
+        if (!TESTANY(OFFLINE_FILE_TYPE_FILTERED | OFFLINE_FILE_TYPE_IFILTERED,
+                     shard->file_type_)) {
+            report_if_false(shard, type_is_instr(shard->prev_instr_.memref.instr.type),
+                            "An unfiltered thread should have at least 1 instruction");
+        }
     }
     if (shard->prev_entry_.marker.type == TRACE_TYPE_MARKER &&
         shard->prev_entry_.marker.marker_type == TRACE_MARKER_TYPE_PHYSICAL_ADDRESS) {
diff --git a/clients/drcachesim/tracer/output.cpp b/clients/drcachesim/tracer/output.cpp
index 6f28e14e0f8..ea4927c9b92 100644
--- a/clients/drcachesim/tracer/output.cpp
+++ b/clients/drcachesim/tracer/output.cpp
@@ -111,6 +111,22 @@ local_instr_count_threshold(uint64 trace_for_instrs)
     }
 }
 
+static bool
+buffer_contains_nontrivial_data(per_thread_t *data)
+{
+    if (op_L0I_filter.get_value()) {
+        return BUF_PTR(data->seg_base) - data->buf_base >
+            static_cast<ssize_t>(data->init_header_size + buf_hdr_slots_size);
+    }
+    byte *buf_ptr = BUF_PTR(data->seg_base);
+    for (byte *mem_ref = data->buf_base + buf_hdr_slots_size; mem_ref < buf_ptr;
+         mem_ref += instru->sizeof_entry()) {
+        if (instru->get_instr_count(mem_ref) > 0)
+            return true;
+    }
+    return false;
+}
+
 // Returns whether we've reached the end of this tracing window.
 static bool
 count_traced_instrs(void *drcontext, uintptr_t toadd, uint64 trace_for_instrs)
@@ -1362,24 +1378,28 @@ exit_thread_io(void *drcontext)
     }
 #endif
 
-    // Append a thread exit marker and output remaining records for this thread
-    // if we are still in a tracing mode or the thread has data from a prior window
-    // that it never wrote out.
-    if (is_in_tracing_mode(tracing_mode.load(std::memory_order_acquire)) ||
-        (has_tracing_windows() &&
-         // If non-split we always want to append a thread exit marker as there
-         // wouldn't be on otherwise (split has one at the end of each window file).
-         (!op_split_windows.get_value() ||
-          // If split, we only need to write if we have data from a prior window.
-          (get_local_window(data) < tracing_window.load(std::memory_order_acquire) &&
-           !is_new_window_buffer_empty(data)))) ||
-        // For attach we switch to BBDUP_MODE_NOP but still need to finalize
-        // each thread.  However, we omit threads that did nothing the entire time
-        // we were attached.
-        (!has_tracing_windows() && align_attach_detach_endpoints() &&
-         (data->bytes_written > 0 ||
-          BUF_PTR(data->seg_base) - data->buf_base >
-              static_cast<ssize_t>(data->init_header_size + buf_hdr_slots_size)))) {
+    // Append a thread exit marker and output remaining records for this thread if it has
+    // data from a prior window that it never wrote out.
+    bool has_prior_window_data = has_tracing_windows() &&
+        // If non-split we always want to append a thread exit marker as there
+        // wouldn't be one otherwise (split has one at the end of each window file).
+        (!op_split_windows.get_value() ||
+         // If split, we only need to write if we have data from a prior window.
+         (get_local_window(data) < tracing_window.load(std::memory_order_acquire) &&
+          !is_new_window_buffer_empty(data)));
+
+    // Also append an exit for non-empty threads (those that wrote buffers out before,
+    // or have a current non-empty buffer).  We completely omit empty threads.
+    bool is_not_empty =
+        (data->bytes_written > 0 || buffer_contains_nontrivial_data(data)) &&
+        // XXX: We may not need any of the conditions below?  Should revisit
+        // whether a current-nop window-up-to-date needs to be excluded here.
+        (is_in_tracing_mode(tracing_mode.load(std::memory_order_acquire)) ||
+         // For attach we switch to BBDUP_MODE_NOP but still need to finalize
+         // each (non-empty) thread.
+         (!has_tracing_windows() && align_attach_detach_endpoints()));
+
+    if (has_prior_window_data || is_not_empty) {
         BUF_PTR(data->seg_base) += instru->append_thread_exit(
             BUF_PTR(data->seg_base), dr_get_thread_id(drcontext));
         process_and_output_buffer(drcontext,