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

Wenlei He via llvm-dev
2020-07-05T06:28:18Z

This sounds very useful. We’ve improved and used
memoro<https://www.youtube.com/watch?v=fm47XsATelI> for memory profiling
and analysis, and we are also looking for ways to leverage memory profile for
PGO/FDO. I think having a common profiling infrastructure for analysis tooling
as well as profile guided optimizations is good design, and having it in LLVM is
also helpful. Very interested in the tooling and optimization that comes after
the profiler.

Two questions:

  *   How does the profiling overhead look? Is that similar to ASAN overhead
from what you’ve seen, which would be higher than PGO instrumentation? Asking
because I’m wondering if any PGO training setup can be used directly for the new
heap profiling.
  *   I’m not familiar with how sanitizer handles stack trace, but for getting
most accurate calling context (use FP rather than dwarf), I guess frame pointer
omission and tail call opt etc. need to be turned off? Is that going to be
implied by -fheapprof?

Thanks,
Wenlei

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Teresa
Johnson via llvm-dev <llvm-dev at lists.llvm.org>
Reply-To: Teresa Johnson <tejohnson at google.com>
Date: Wednesday, June 24, 2020 at 4:58 PM
To: llvm-dev <llvm-dev at lists.llvm.org>, Kostya Serebryany <kcc at
google.com>, Evgenii Stepanov <eugenis at google.com>, Vitaly Buka
<vitalybuka at google.com>
Cc: David Li <davidxl at google.com>
Subject: [llvm-dev] RFC: Sanitizer-based Heap Profiler

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://urldefense.proofpoint.com/v2/url?u=https-3A__en.wikipedia.org_wiki_Shadow-5Fmemory&d=DwMFaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=KfYo542rDdZQGClmgz-RBw&m=f45oT3WLypO1yblv9KNkPd-rl8jlBp761Hhvev27S8M&s=iIirMZSYnDlGIjY8PZjJprWckHx7QhmKUQKcb1URBFY&e=>,
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<mailto:tejohnson at google.com> |

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