[llvm-dev] RFC: Sanitizer-based Heap Profiler

[Teresa Johnson via llvm-dev]

Hi all,

I've included an RFC for a heap profiler design I've been working on in
conjunction with David Li. Please send any questions or feedback. For
sanitizer folks, one area of feedback is on refactoring some of the *ASAN
shadow setup code (see the Shadow Memory section).

Thanks,
Teresa


RFC: Sanitizer-based Heap Profiler
Summary

This document provides an overview of an LLVM Sanitizer-based heap profiler
design.
Motivation

The objective of heap memory profiling is to collect critical runtime
information associated with heap memory references and information on heap
memory allocations. The profile information will be used first for tooling,
and subsequently to guide the compiler optimizer and allocation runtime to
layout heap objects with improved spatial locality. As a  result, DTLB and
cache utilization will be improved, and program IPC (performance) will be
increased due to reduced TLB and cache misses. More details on the heap
profile guided optimizations will be shared in the future.
Overview

The profiler is based on compiler inserted instrumentation of load and
store accesses, and utilizes runtime support to monitor heap allocations
and profile data. The target consumer of the heap memory profile
information is initially tooling and ultimately automatic data layout
optimizations performed by the compiler and/or allocation runtime (with the
support of new allocation runtime APIs).

Each memory address is mapped to Shadow Memory
<https://en.wikipedia.org/wiki/Shadow_memory>, similar to the approach used
by the Address Sanitizer
<https://github.com/google/sanitizers/wiki/AddressSanitizer> (ASAN). Unlike
ASAN, which maps each 8 bytes of memory to 1 byte of shadow, the heap
profiler maps 64 bytes of memory to 8 bytes of shadow. The shadow location
implements the profile counter (incremented on accesses to the
corresponding memory). This granularity was chosen to help avoid counter
overflow, but it may be possible to consider mapping 32-bytes to 4 bytes.
To avoid aliasing of shadow memory for different allocations, we must
choose a minimum alignment carefully. As discussed further below, we can
attain a 32-byte minimum alignment, instead of a 64-byte alignment, by
storing necessary heap information for each allocation in a 32-byte header
block.

The compiler instruments each load and store to increment the associated
shadow memory counter, in order to determine hotness.

The heap profiler runtime is responsible for tracking allocations and
deallocations, including the stack at each allocation, and information such
as the allocation size and other statistics. I have implemented a prototype
built using a stripped down and modified version of ASAN, however this will
be a separate library utilizing sanitizer_common components.
Compiler

A simple HeapProfiler instrumentation pass instruments interesting memory
accesses (loads, stores, atomics), with a simple load, increment, store of
the associated shadow memory location (computed via a mask and shift to do
the mapping of 64 bytes to 8 byte shadow, and add of the shadow offset).
The handling is very similar to and based off of the ASAN instrumentation
pass, with slightly different instrumentation code.

Various techniques can be used to reduce the overhead, by aggressively
coalescing counter updates (e.g. given the 32-byte alignment, accesses
known to be in the same 32-byte block, or across possible aliases since we
don’t care about the dereferenced values).

Additionally, the Clang driver needs to set up to link with the runtime
library, much as it does with the sanitizers.

A -fheapprof option is added to enable the instrumentation pass and runtime
library linking. Similar to -fprofile-generate, -fheapprof will accept an
argument specifying the directory in which to write the profile.
Runtime

The heap profiler runtime is responsible for tracking and reporting
information about heap allocations and accesses, aggregated by allocation
calling context. For example, the hotness, lifetime, and cpu affinity.

A new heapprof library will be created within compiler-rt. It will leverage
support within sanitizer_common, which already contains facilities like
stack context tracking, needed by the heap profiler.
Shadow Memory

There are some basic facilities in sanitizer_common for mmap’ing the shadow
memory, but most of the existing setup lives in the ASAN and HWASAN
libraries. In the case of ASAN, there is support for both statically
assigned shadow offsets (the default on most platforms), and for
dynamically assigned shadow memory (implemented for Windows and currently
also used for Android and iOS). According to kcc, recent experiments show
that the performance with a dynamic shadow is close to that with a static
mapping. In fact, that is the only approach currently used by HWASAN. Given
the simplicity, the heap profiler will be implemented with a dynamic shadow
as well.

There are a number of functions in ASAN and HWASAN related to setup of the
shadow that are duplicated but very nearly identical, at least for linux
(which seems to be the only OS flavor currently supported for HWASAN). E.g.
ReserveShadowMemoryRange, ProtectGap, and FindDynamicShadowStart (in ASAN
there is another nearly identical copy in PremapShadow, used by Android,
whereas in HW ASAN the premap handling is already commoned with the
non-premap handling). Rather than make yet another copy of these
mechanisms, I propose refactoring them into sanitizer_common versions. Like
HWASAN, the initial version of the heap profiler will be supported for
linux only, but other OSes can be added as needed similar to ASAN.
StackTrace and StackDepot

The sanitizer already contains support for obtaining and representing a
stack trace in a StackTrace object, and storing it in the StackDepot which
“efficiently stores huge amounts of stack traces”. This is in the
sanitizer_common subdirectory and the support is shared by ASAN and
ThreadSanitizer. The StackDepot is essentially an unbounded hash table,
where each StackTrace is assigned a unique id. ASAN stores this id in the
alloc_context_id field in each ChunkHeader (in the redzone preceding each
allocation). Additionally, there is support for symbolizing and printing
StackTrace objects.
ChunkHeader

The heap profiler needs to track several pieces of information for each
allocation. Given the mapping of 64-bytes to 8-bytes shadow, we can achieve
a minimum of 32-byte alignment by holding this information in a 32-byte
header block preceding each allocation.

In ASAN, each allocation is preceded by a 16-byte ChunkHeader. It contains
information about the current allocation state, user requested size,
allocation and free thread ids, the allocation context id (representing the
call stack at allocation, assigned by the StackDepot as described above),
and misc other bookkeeping. For heap profiling, this will be converted to a
32-byte header block.

Note that we could instead use the metadata section, similar to other
sanitizers, which is stored in a separate location. However, as described
above, storing the header block with each allocation enables 32-byte
alignment without aliasing shadow counters for the same 64 bytes of memory.

In the prototype heap profiler implementation, the header contains the
following fields:

// Should be 32 bytes

struct ChunkHeader {

  // 1-st 4 bytes

  // Carry over from ASAN (available, allocated, quarantined). Will be

  // reduced to 1 bit (available or allocated).

  u32 chunk_state       : 8;

  // Carry over from ASAN. Used to determine the start of user allocation.

  u32 from_memalign     : 1;

  // 23 bits available

  // 2-nd 4 bytes

  // Carry over from ASAN (comment copied verbatim).

  // This field is used for small sizes. For large sizes it is equal to

  // SizeClassMap::kMaxSize and the actual size is stored in the

  // SecondaryAllocator's metadata.

  u32 user_requested_size : 29;

  // 3-rd 4 bytes

  u32 cpu_id; // Allocation cpu id

  // 4-th 4 bytes

  // Allocation timestamp in ms from a baseline timestamp computed at

  // the start of profiling (to keep this within 32 bits).

  u32 timestamp_ms;

  // 5-th and 6-th 4 bytes

  // Carry over from ASAN. Used to identify allocation stack trace.

  u64 alloc_context_id;

  // 7-th and 8-th 4 bytes

  // UNIMPLEMENTED in prototype - needs instrumentation and IR support.

  u64 data_type_id; // hash of type name

};

As noted, the chunk state can be reduced to a single bit (no need for
quarantined memory in the heap profiler). The header contains a placeholder
for the data type hash, which is not yet implemented as it needs
instrumentation and IR support.
Heap Info Block (HIB)

On a deallocation, information from the corresponding shadow block(s) and
header are recorded in a Heap Info Block (HIB) object. The access count is
computed from the shadow memory locations for the allocation, as well as
the percentage of accessed 64-byte blocks (i.e. the percentage of non-zero
8-byte shadow locations for the whole allocation). Other information such
as the deallocation timestamp (for lifetime computation) and deallocation
cpu id (to determine migrations) are recorded along with the information in
the chunk header recorded on allocation.

The prototyped HIB object tracks the following:

struct HeapInfoBlock {

  // Total allocations at this stack context

  u32 alloc_count;

  // Access count computed from all allocated 64-byte blocks (track total

  // across all allocations, and the min and max).

  u64 total_access_count, min_access_count, max_access_count;

  // Allocated size (track total across all allocations, and the min and
max).

  u64 total_size;

  u32 min_size, max_size;

  // Lifetime (track total across all allocations, and the min and max).

  u64 total_lifetime;

  u32 min_lifetime, max_lifetime;

  // Percent utilization of allocated 64-byte blocks (track total

  // across all allocations, and the min and max). The utilization is

  // defined as the percentage of 8-byte shadow counters corresponding to

  // the full allocation that are non-zero.

  u64 total_percent_utilized;

  u32 min_percent_utilized, max_percent_utilized;

  // Allocation and deallocation timestamps from the most recent merge into

  // the table with this stack context.

  u32 alloc_timestamp, dealloc_timestamp;

  // Allocation and deallocation cpu ids from the most recent merge into

  // the table with this stack context.

  u32 alloc_cpu_id, dealloc_cpu_id;

  // Count of allocations at this stack context that had a different

  // allocation and deallocation cpu id.

  u32 num_migrated_cpu;

  // Number of times the lifetime of the entry being merged had its lifetime

  // overlap with the previous entry merged with this stack context (by

  // comparing the new alloc/dealloc timestamp with the one last recorded in

  // the entry in the table.

  u32 num_lifetime_overlaps;

  // Number of times the alloc/dealloc cpu of the entry being merged was the

  // same as that of the previous entry merged with this stack context

  u32 num_same_alloc_cpu;

  u32 num_same_dealloc_cpu;

  // Hash of type name (UNIMPLEMENTED). This needs instrumentation support
and

  // possibly IR changes.

  u64 data_type_id;

}
HIB Table

The Heap Info Block Table, which is a multi-way associative cache, holds
HIB objects from deallocated objects. It is indexed by the stack allocation
context id from the chunk header, and currently utilizes a simple mod with
a prime number close to a power of two as the hash (because of the way the
stack context ids are assigned, a mod of a power of two performs very
poorly). Thus far, only 4-way associativity has been evaluated.

HIB entries are added or merged into the HIB Table on each deallocation. If
an entry with a matching stack alloc context id is found in the Table, the
newly deallocated information is merged into the existing entry. Each HIB
Table entry currently tracks the min, max and total value of the various
fields for use in computing and reporting the min, max and average when the
Table is ultimately dumped.

If no entry with a matching stack alloc context id is found, a new entry is
created. If this causes an eviction, the evicted entry is dumped
immediately (by default to stderr, otherwise to a specified report file).
Later post processing can merge dumped entries with the same stack alloc
context id.
Initialization

For ASAN, an __asan_init function initializes the memory allocation
tracking support, and the ASAN instrumentation pass in LLVM creates a
global constructor to invoke it. The heap profiler prototype adds a new
__heapprof_init function, which performs heap profile specific
initialization, and the heap profile instrumentation pass calls this new
init function instead by a generated global constructor. It currently
additionally invokes __asan_init since we are leveraging a modified ASAN
runtime. Eventually, this should be changed to initialize refactored common
support.

Note that __asan init is also placed in the .preinit_array when it is
available, so it is invoked even earlier than global constructors.
Currently, it is not possible to do this for __heapprof_init, as it calls
timespec_get in order to get a baseline timestamp (as described in the
ChunkHeader comments the timestamps (ms) are actually offsets from the
baseline timestamp, in order to fit into 32 bits), and system calls cannot
be made that early (dl_init is not complete). Since the constructor
priority is 1, it should be executed early enough that there are very few
allocations before it runs, and likely the best solution is to simply
ignore any allocations before initialization.
Dumping

For the prototype, the profile is dumped as text with a compact raw format
to limit its size. Ultimately it should be dumped in a more compact binary
format (i.e. into a different section of the raw instrumentation based
profile, with llvm-profdata performing post-processing) which is TBD.
HIB Dumping

As noted earlier, HIB Table entries are created as memory is deallocated.
At the end of the run (or whenever dumping is requested, discussed later),
HIB entries need to be created for allocations that are still live.
Conveniently, the sanitizer allocator already contains a mechanism to walk
through all chunks of memory it is tracking (ForEachChunk). The heap
profiler simply looks for all chunks with a chunk state of allocated, and
creates a HIB the same as would be done on deallocation, adding each to the
table.

A HIB Table mechanism for printing each entry is then invoked.

By default, the dumping occurs:

   -

   on evictions
   -

   full table at exit (when the static Allocator object is destructed)


For running in a load testing scenario, we will want to add a mechanism to
provoke finalization (merging currently live allocations) and dumping of
the HIB Table before exit. This would be similar to the __llvm_profile_dump
facility used for normal PGO counter dumping.
Stack Trace Dumping

There is existing support for dumping symbolized StackTrace objects. A
wrapper to dump all StackTrace objects in the StackDepot will be added.
This new interface is invoked just after the HIB Table is dumped (on exit
or via dumping interface).
Memory Map Dumping

In cases where we may want to symbolize as a post processing step, we may
need the memory map (from /proc/self/smaps). Specifically, this is needed
to symbolize binaries using ASLR (Address Space Layout Randomization).
There is already support for reading this file and dumping it to the
specified report output file (DumpProcessMap()). This is invoked when the
profile output file is initialized (HIB Table construction), so that the
memory map is available at the top of the raw profile.
Current Status and Next Steps

As mentioned earlier, I have a working prototype based on a simplified
stripped down version of ASAN. My current plan is to do the following:

   1.

   Refactor out some of the shadow setup code common between ASAN and
   HWASAN into sanitizer_common.
   2.

   Rework my prototype into a separate heapprof library in compiler-rt,
   using sanitizer_common support where possible, and send patches for review.
   3.

   Send patches for the heap profiler instrumentation pass and related
   clang options.
   4.

   Design/implement binary profile format


-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |
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