gh-135953: Reduce memory usage of stack collectors (#138875)

The stack collector base class keeps all frames until export() is
called, which causes significant unnecessary memory usage. Instead, we
can process the frames on the fly in the collect call by dispatching the
aggregation logic to the subclass through the process_frames method.

Co-authored-by: Pablo Galindo Salgado <pablogsal@gmail.com>

Lib/profiling/sampling/__init__.py

@@ -7,5 +7,6 @@
 from .collector import Collector
 from .pstats_collector import PstatsCollector
 from .stack_collector import CollapsedStackCollector
+from .string_table import StringTable
 
-__all__ = ("Collector", "PstatsCollector", "CollapsedStackCollector")
+__all__ = ("Collector", "PstatsCollector", "CollapsedStackCollector", "StringTable")

Lib/profiling/sampling/flamegraph.js

@@ -1,5 +1,50 @@
 const EMBEDDED_DATA = {{FLAMEGRAPH_DATA}};
 
+// Global string table for resolving string indices
+let stringTable = [];
+
+// Function to resolve string indices to actual strings
+function resolveString(index) {
+    if (typeof index === 'number' && index >= 0 && index < stringTable.length) {
+        return stringTable[index];
+    }
+    // Fallback for non-indexed strings or invalid indices
+    return String(index);
+}
+
+// Function to recursively resolve all string indices in flamegraph data
+function resolveStringIndices(node) {
+    if (!node) return node;
+
+    // Create a copy to avoid mutating the original
+    const resolved = { ...node };
+
+    // Resolve string fields
+    if (typeof resolved.name === 'number') {
+        resolved.name = resolveString(resolved.name);
+    }
+    if (typeof resolved.filename === 'number') {
+        resolved.filename = resolveString(resolved.filename);
+    }
+    if (typeof resolved.funcname === 'number') {
+        resolved.funcname = resolveString(resolved.funcname);
+    }
+
+    // Resolve source lines if present
+    if (Array.isArray(resolved.source)) {
+        resolved.source = resolved.source.map(index =>
+            typeof index === 'number' ? resolveString(index) : index
+        );
+    }
+
+    // Recursively resolve children
+    if (Array.isArray(resolved.children)) {
+        resolved.children = resolved.children.map(child => resolveStringIndices(child));
+    }
+
+    return resolved;
+}
+
 // Python color palette - cold to hot
 const pythonColors = [
   "#fff4bf", // Coldest - light yellow (<1%)
@@ -100,6 +145,10 @@ function createPythonTooltip(data) {
         </div>`;
     }
 
+    // Resolve strings for display
+    const funcname = resolveString(d.data.funcname) || resolveString(d.data.name);
+    const filename = resolveString(d.data.filename) || "";
+
     const tooltipHTML = `
       <div>
         <div style="color: #3776ab; font-weight: 600; font-size: 16px;
@@ -257,9 +306,9 @@ function updateSearchHighlight(searchTerm, searchInput) {
     let matchCount = 0;
     d3.selectAll("#chart rect").each(function (d) {
       if (d && d.data) {
-        const name = d.data.name || "";
+        const name = resolveString(d.data.name) || "";
-        const funcname = d.data.funcname || "";
+        const funcname = resolveString(d.data.funcname) || "";
-        const filename = d.data.filename || "";
+        const filename = resolveString(d.data.filename) || "";
         const term = searchTerm.toLowerCase();
         const matches =
           name.toLowerCase().includes(term) ||
@@ -317,12 +366,20 @@ function handleResize(chart, data) {
 
 function initFlamegraph() {
   ensureLibraryLoaded();
-  const tooltip = createPythonTooltip(EMBEDDED_DATA);
+
-  const chart = createFlamegraph(tooltip, EMBEDDED_DATA.value);
+  // Extract string table if present and resolve string indices
-  renderFlamegraph(chart, EMBEDDED_DATA);
+  let processedData = EMBEDDED_DATA;
+  if (EMBEDDED_DATA.strings) {
+    stringTable = EMBEDDED_DATA.strings;
+    processedData = resolveStringIndices(EMBEDDED_DATA);
+  }
+
+  const tooltip = createPythonTooltip(processedData);
+  const chart = createFlamegraph(tooltip, processedData.value);
+  renderFlamegraph(chart, processedData);
   attachPanelControls();
   initSearchHandlers();
-  handleResize(chart, EMBEDDED_DATA);
+  handleResize(chart, processedData);
 }
 
 if (document.readyState === "loading") {
@@ -338,7 +395,10 @@ function populateStats(data) {
   const functionMap = new Map();
 
   function collectFunctions(node) {
-    if (node.filename && node.funcname) {
+    const filename = resolveString(node.filename);
+    const funcname = resolveString(node.funcname);
+
+    if (filename && funcname) {
       // Calculate direct samples (this node's value minus children's values)
       let childrenValue = 0;
       if (node.children) {
@@ -347,7 +407,7 @@ function populateStats(data) {
       const directSamples = Math.max(0, node.value - childrenValue);
 
       // Use file:line:funcname as key to ensure uniqueness
-      const funcKey = `${node.filename}:${node.lineno || '?'}:${node.funcname}`;
+      const funcKey = `${filename}:${node.lineno || '?'}:${funcname}`;
 
       if (functionMap.has(funcKey)) {
         const existing = functionMap.get(funcKey);
@@ -355,15 +415,15 @@ function populateStats(data) {
         existing.directPercent = (existing.directSamples / totalSamples) * 100;
         // Keep the most representative file/line (the one with more samples)
         if (directSamples > existing.maxSingleSamples) {
-          existing.filename = node.filename;
+          existing.filename = filename;
           existing.lineno = node.lineno || '?';
           existing.maxSingleSamples = directSamples;
         }
       } else {
         functionMap.set(funcKey, {
-          filename: node.filename,
+          filename: filename,
           lineno: node.lineno || '?',
-          funcname: node.funcname,
+          funcname: funcname,
           directSamples,
           directPercent: (directSamples / totalSamples) * 100,
           maxSingleSamples: directSamples

Lib/profiling/sampling/stack_collector.py

@@ -7,51 +7,51 @@
 import os
 
 from .collector import Collector
+from .string_table import StringTable
 
 
 class StackTraceCollector(Collector):
-    def __init__(self):
-        self.call_trees = []
-        self.function_samples = collections.defaultdict(int)
-
-    def _process_frames(self, frames):
-        """Process a single thread's frame stack."""
-        if not frames:
-            return
-
-        # Store the complete call stack (reverse order - root first)
-        call_tree = list(reversed(frames))
-        self.call_trees.append(call_tree)
-
-        # Count samples per function
-        for frame in frames:
-            self.function_samples[frame] += 1
-
     def collect(self, stack_frames):
         for frames in self._iter_all_frames(stack_frames):
-            self._process_frames(frames)
+            if not frames:
+                continue
+            self.process_frames(frames)
+
+    def process_frames(self, frames):
+        pass
 
 
 class CollapsedStackCollector(StackTraceCollector):
+    def __init__(self):
+        self.stack_counter = collections.Counter()
+
+    def process_frames(self, frames):
+        call_tree = tuple(reversed(frames))
+        self.stack_counter[call_tree] += 1
+
     def export(self, filename):
-        stack_counter = collections.Counter()
+        lines = []
-        for call_tree in self.call_trees:
+        for call_tree, count in self.stack_counter.items():
-            # Call tree is already in root->leaf order
             stack_str = ";".join(
                 f"{os.path.basename(f[0])}:{f[2]}:{f[1]}" for f in call_tree
             )
-            stack_counter[stack_str] += 1
+            lines.append((stack_str, count))
+
+        lines.sort(key=lambda x: (-x[1], x[0]))
 
         with open(filename, "w") as f:
-            for stack, count in stack_counter.items():
+            for stack, count in lines:
                 f.write(f"{stack} {count}\n")
         print(f"Collapsed stack output written to {filename}")
 
 
 class FlamegraphCollector(StackTraceCollector):
     def __init__(self):
-        super().__init__()
         self.stats = {}
+        self._root = {"samples": 0, "children": {}}
+        self._total_samples = 0
+        self._func_intern = {}
+        self._string_table = StringTable()
 
     def set_stats(self, sample_interval_usec, duration_sec, sample_rate, error_rate=None):
         """Set profiling statistics to include in flamegraph data."""
@@ -65,11 +65,13 @@ def set_stats(self, sample_interval_usec, duration_sec, sample_rate, error_rate=
     def export(self, filename):
         flamegraph_data = self._convert_to_flamegraph_format()
 
-        # Debug output
+        # Debug output with string table statistics
         num_functions = len(flamegraph_data.get("children", []))
         total_time = flamegraph_data.get("value", 0)
+        string_count = len(self._string_table)
         print(
-            f"Flamegraph data: {num_functions} root functions, total samples: {total_time}"
+            f"Flamegraph data: {num_functions} root functions, total samples: {total_time}, "
+            f"{string_count} unique strings"
         )
 
         if num_functions == 0:
@@ -98,105 +100,105 @@ def _format_function_name(func):
         return f"{funcname} ({filename}:{lineno})"
 
     def _convert_to_flamegraph_format(self):
-        """Convert call trees to d3-flamegraph format with optimized hierarchy building"""
+        """Convert aggregated trie to d3-flamegraph format with string table optimization."""
-        if not self.call_trees:
+        if self._total_samples == 0:
-            return {"name": "No Data", "value": 0, "children": []}
+            return {
-
+                "name": self._string_table.intern("No Data"),
-        unique_functions = set()
+                "value": 0,
-        for call_tree in self.call_trees:
-            unique_functions.update(call_tree)
-
-        func_to_name = {
-            func: self._format_function_name(func) for func in unique_functions
-        }
-
-        root = {"name": "root", "children": {}, "samples": 0}
-
-        for call_tree in self.call_trees:
-            current_node = root
-            current_node["samples"] += 1
-
-            for func in call_tree:
-                func_name = func_to_name[func]  # Use pre-computed name
-
-                if func_name not in current_node["children"]:
-                    current_node["children"][func_name] = {
-                        "name": func_name,
-                        "func": func,
-                        "children": {},
-                        "samples": 0,
-                        "filename": func[0],
-                        "lineno": func[1],
-                        "funcname": func[2],
-                    }
-
-                current_node = current_node["children"][func_name]
-                current_node["samples"] += 1
-
-        def convert_node(node, min_samples=1):
-            if node["samples"] < min_samples:
-                return None
-
-            source_code = None
-            if "func" in node:
-                source_code = self._get_source_lines(node["func"])
-
-            result = {
-                "name": node["name"],
-                "value": node["samples"],
                 "children": [],
+                "strings": self._string_table.get_strings()
             }
 
-            if "filename" in node:
+        def convert_children(children, min_samples):
-                result.update(
+            out = []
-                    {
+            for func, node in children.items():
-                        "filename": node["filename"],
+                samples = node["samples"]
-                        "lineno": node["lineno"],
+                if samples < min_samples:
-                        "funcname": node["funcname"],
+                    continue
-                    }
+
+                # Intern all string components for maximum efficiency
+                filename_idx = self._string_table.intern(func[0])
+                funcname_idx = self._string_table.intern(func[2])
+                name_idx = self._string_table.intern(self._format_function_name(func))
+
+                child_entry = {
+                    "name": name_idx,
+                    "value": samples,
+                    "children": [],
+                    "filename": filename_idx,
+                    "lineno": func[1],
+                    "funcname": funcname_idx,
+                }
+
+                source = self._get_source_lines(func)
+                if source:
+                    # Intern source lines for memory efficiency
+                    source_indices = [self._string_table.intern(line) for line in source]
+                    child_entry["source"] = source_indices
+
+                # Recurse
+                child_entry["children"] = convert_children(
+                    node["children"], min_samples
                 )
+                out.append(child_entry)
 
-            if source_code:
+            # Sort by value (descending) then by name index for consistent ordering
-                result["source"] = source_code
+            out.sort(key=lambda x: (-x["value"], x["name"]))
-
+            return out
-            # Recursively convert children
-            child_nodes = []
-            for child_name, child_node in node["children"].items():
-                child_result = convert_node(child_node, min_samples)
-                if child_result:
-                    child_nodes.append(child_result)
-
-            # Sort children by sample count (descending)
-            child_nodes.sort(key=lambda x: x["value"], reverse=True)
-            result["children"] = child_nodes
-
-            return result
 
         # Filter out very small functions (less than 0.1% of total samples)
-        total_samples = len(self.call_trees)
+        total_samples = self._total_samples
         min_samples = max(1, int(total_samples * 0.001))
 
-        converted_root = convert_node(root, min_samples)
+        root_children = convert_children(self._root["children"], min_samples)
-
+        if not root_children:
-        if not converted_root or not converted_root["children"]:
+            return {
-            return {"name": "No significant data", "value": 0, "children": []}
+                "name": self._string_table.intern("No significant data"),
+                "value": 0,
+                "children": [],
+                "strings": self._string_table.get_strings()
+            }
 
         # If we only have one root child, make it the root to avoid redundant level
-        if len(converted_root["children"]) == 1:
+        if len(root_children) == 1:
-            main_child = converted_root["children"][0]
+            main_child = root_children[0]
-            main_child["name"] = f"Program Root: {main_child['name']}"
+            # Update the name to indicate it's the program root
+            old_name = self._string_table.get_string(main_child["name"])
+            new_name = f"Program Root: {old_name}"
+            main_child["name"] = self._string_table.intern(new_name)
             main_child["stats"] = self.stats
+            main_child["strings"] = self._string_table.get_strings()
             return main_child
 
-        converted_root["name"] = "Program Root"
+        return {
-        converted_root["stats"] = self.stats
+            "name": self._string_table.intern("Program Root"),
-        return converted_root
+            "value": total_samples,
+            "children": root_children,
+            "stats": self.stats,
+            "strings": self._string_table.get_strings()
+        }
+
+    def process_frames(self, frames):
+        # Reverse to root->leaf
+        call_tree = reversed(frames)
+        self._root["samples"] += 1
+        self._total_samples += 1
+
+        current = self._root
+        for func in call_tree:
+            func = self._func_intern.setdefault(func, func)
+            children = current["children"]
+            node = children.get(func)
+            if node is None:
+                node = {"samples": 0, "children": {}}
+                children[func] = node
+            node["samples"] += 1
+            current = node
 
     def _get_source_lines(self, func):
-        filename, lineno, funcname = func
+        filename, lineno, _ = func
 
         try:
-            # Get several lines around the function definition
             lines = []
             start_line = max(1, lineno - 2)
             end_line = lineno + 3
@@ -210,7 +212,6 @@ def _get_source_lines(self, func):
             return lines if lines else None
 
         except Exception:
-            # If we can't get source code, return None
             return None
 
     def _create_flamegraph_html(self, data):

Lib/profiling/sampling/string_table.py

@@ -0,0 +1,53 @@
+"""String table implementation for memory-efficient string storage in profiling data."""
+
+class StringTable:
+    """A string table for interning strings and reducing memory usage."""
+
+    def __init__(self):
+        self._strings = []
+        self._string_to_index = {}
+
+    def intern(self, string):
+        """Intern a string and return its index.
+
+        Args:
+            string: The string to intern
+
+        Returns:
+            int: The index of the string in the table
+        """
+        if not isinstance(string, str):
+            string = str(string)
+
+        if string in self._string_to_index:
+            return self._string_to_index[string]
+
+        index = len(self._strings)
+        self._strings.append(string)
+        self._string_to_index[string] = index
+        return index
+
+    def get_string(self, index):
+        """Get a string by its index.
+
+        Args:
+            index: The index of the string
+
+        Returns:
+            str: The string at the given index, or empty string if invalid
+        """
+        if 0 <= index < len(self._strings):
+            return self._strings[index]
+        return ""
+
+    def get_strings(self):
+        """Get the list of all strings in the table.
+
+        Returns:
+            list: A copy of the strings list
+        """
+        return self._strings.copy()
+
+    def __len__(self):
+        """Return the number of strings in the table."""
+        return len(self._strings)

Lib/test/test_profiling/test_sampling_profiler.py

@@ -272,25 +272,26 @@ def test_collapsed_stack_collector_with_empty_and_deep_stacks(self):
 
         # Test with empty frames
         collector.collect([])
-        self.assertEqual(len(collector.call_trees), 0)
+        self.assertEqual(len(collector.stack_counter), 0)
 
         # Test with single frame stack
         test_frames = [MockInterpreterInfo(0, [MockThreadInfo(1, [("file.py", 10, "func")])])]
         collector.collect(test_frames)
-        self.assertEqual(len(collector.call_trees), 1)
+        self.assertEqual(len(collector.stack_counter), 1)
-        self.assertEqual(collector.call_trees[0], [("file.py", 10, "func")])
+        ((path,), count), = collector.stack_counter.items()
+        self.assertEqual(path, ("file.py", 10, "func"))
+        self.assertEqual(count, 1)
 
         # Test with very deep stack
         deep_stack = [(f"file{i}.py", i, f"func{i}") for i in range(100)]
         test_frames = [MockInterpreterInfo(0, [MockThreadInfo(1, deep_stack)])]
         collector = CollapsedStackCollector()
         collector.collect(test_frames)
-        self.assertEqual(len(collector.call_trees[0]), 100)
+        # One aggregated path with 100 frames (reversed)
-        # Check it's properly reversed
+        (path_tuple,), = (collector.stack_counter.keys(),)
-        self.assertEqual(
+        self.assertEqual(len(path_tuple), 100)
-            collector.call_trees[0][0], ("file99.py", 99, "func99")
+        self.assertEqual(path_tuple[0], ("file99.py", 99, "func99"))
-        )
+        self.assertEqual(path_tuple[-1], ("file0.py", 0, "func0"))
-        self.assertEqual(collector.call_trees[0][-1], ("file0.py", 0, "func0"))
 
     def test_pstats_collector_basic(self):
         """Test basic PstatsCollector functionality."""
@@ -382,8 +383,7 @@ def test_collapsed_stack_collector_basic(self):
         collector = CollapsedStackCollector()
 
         # Test empty state
-        self.assertEqual(len(collector.call_trees), 0)
+        self.assertEqual(len(collector.stack_counter), 0)
-        self.assertEqual(len(collector.function_samples), 0)
 
         # Test collecting sample data
         test_frames = [
@@ -391,18 +391,12 @@ def test_collapsed_stack_collector_basic(self):
         ]
         collector.collect(test_frames)
 
-        # Should store call tree (reversed)
+        # Should store one reversed path
-        self.assertEqual(len(collector.call_trees), 1)
+        self.assertEqual(len(collector.stack_counter), 1)
-        expected_tree = [("file.py", 20, "func2"), ("file.py", 10, "func1")]
+        (path, count), = collector.stack_counter.items()
-        self.assertEqual(collector.call_trees[0], expected_tree)
+        expected_tree = (("file.py", 20, "func2"), ("file.py", 10, "func1"))
-
+        self.assertEqual(path, expected_tree)
-        # Should count function samples
+        self.assertEqual(count, 1)
-        self.assertEqual(
-            collector.function_samples[("file.py", 10, "func1")], 1
-        )
-        self.assertEqual(
-            collector.function_samples[("file.py", 20, "func2")], 1
-        )
 
     def test_collapsed_stack_collector_export(self):
         collapsed_out = tempfile.NamedTemporaryFile(delete=False)
@@ -441,9 +435,13 @@ def test_flamegraph_collector_basic(self):
         """Test basic FlamegraphCollector functionality."""
         collector = FlamegraphCollector()
 
-        # Test empty state (inherits from StackTraceCollector)
+        # Empty collector should produce 'No Data'
-        self.assertEqual(len(collector.call_trees), 0)
+        data = collector._convert_to_flamegraph_format()
-        self.assertEqual(len(collector.function_samples), 0)
+        # With string table, name is now an index - resolve it using the strings array
+        strings = data.get("strings", [])
+        name_index = data.get("name", 0)
+        resolved_name = strings[name_index] if isinstance(name_index, int) and 0 <= name_index < len(strings) else str(name_index)
+        self.assertIn(resolved_name, ("No Data", "No significant data"))
 
         # Test collecting sample data
         test_frames = [
@@ -454,18 +452,22 @@ def test_flamegraph_collector_basic(self):
         ]
         collector.collect(test_frames)
 
-        # Should store call tree (reversed)
+        # Convert and verify structure: func2 -> func1 with counts = 1
-        self.assertEqual(len(collector.call_trees), 1)
+        data = collector._convert_to_flamegraph_format()
-        expected_tree = [("file.py", 20, "func2"), ("file.py", 10, "func1")]
+        # Expect promotion: root is the single child (func2), with func1 as its only child
-        self.assertEqual(collector.call_trees[0], expected_tree)
+        strings = data.get("strings", [])
-
+        name_index = data.get("name", 0)
-        # Should count function samples
+        name = strings[name_index] if isinstance(name_index, int) and 0 <= name_index < len(strings) else str(name_index)
-        self.assertEqual(
+        self.assertIsInstance(name, str)
-            collector.function_samples[("file.py", 10, "func1")], 1
+        self.assertTrue(name.startswith("Program Root: "))
-        )
+        self.assertIn("func2 (file.py:20)", name)  # formatted name
-        self.assertEqual(
+        children = data.get("children", [])
-            collector.function_samples[("file.py", 20, "func2")], 1
+        self.assertEqual(len(children), 1)
-        )
+        child = children[0]
+        child_name_index = child.get("name", 0)
+        child_name = strings[child_name_index] if isinstance(child_name_index, int) and 0 <= child_name_index < len(strings) else str(child_name_index)
+        self.assertIn("func1 (file.py:10)", child_name)  # formatted name
+        self.assertEqual(child["value"], 1)
 
     def test_flamegraph_collector_export(self):
         """Test flamegraph HTML export functionality."""
@@ -1508,28 +1510,29 @@ def test_collapsed_stack_with_recursion(self):
         for frames in recursive_frames:
             collector.collect([frames])
 
-        # Should capture both call trees
+        # Should capture both call paths
-        self.assertEqual(len(collector.call_trees), 2)
+        self.assertEqual(len(collector.stack_counter), 2)
 
-        # First tree should be longer (deeper recursion)
+        # First path should be longer (deeper recursion) than the second
-        tree1 = collector.call_trees[0]
+        paths = list(collector.stack_counter.keys())
-        tree2 = collector.call_trees[1]
+        lengths = [len(p) for p in paths]
-
+        self.assertNotEqual(lengths[0], lengths[1])
-        # Trees should be different lengths due to different recursion depths
-        self.assertNotEqual(len(tree1), len(tree2))
 
         # Both should contain factorial calls
-        self.assertTrue(any("factorial" in str(frame) for frame in tree1))
+        self.assertTrue(any(any(f[2] == "factorial" for f in p) for p in paths))
-        self.assertTrue(any("factorial" in str(frame) for frame in tree2))
 
-        # Function samples should count all occurrences
+        # Verify total occurrences via aggregation
         factorial_key = ("factorial.py", 10, "factorial")
         main_key = ("main.py", 5, "main")
 
-        # factorial appears 5 times total (3 + 2)
+        def total_occurrences(func):
-        self.assertEqual(collector.function_samples[factorial_key], 5)
+            total = 0
-        # main appears 2 times total
+            for path, count in collector.stack_counter.items():
-        self.assertEqual(collector.function_samples[main_key], 2)
+                total += sum(1 for f in path if f == func) * count
+            return total
+
+        self.assertEqual(total_occurrences(factorial_key), 5)
+        self.assertEqual(total_occurrences(main_key), 2)
 
 
 @requires_subprocess()