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

Teresa Johnson via llvm-dev
2020-07-09T19:36:07Z

On Wed, Jul 8, 2020 at 6:30 PM Kostya Serebryany <kcc at google.com>
wrote:

>
>
> On Wed, Jun 24, 2020 at 4:58 PM Teresa Johnson <tejohnson at
google.com>
> wrote:
>
>> 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).
>>
>
> I have no strong opinion on the refactoring of the shadow set up code.
> Getting such code in one common place may or may not make the future
> maintenance cheaper.
>

It was partially motivated by my looking to see what version of shadow
setup to use, as they initially looked somewhat different, only to discover
after carefully tracing through all the values that they were essentially
identical but just structured differently (at least between asan and
hwasan). This is somewhat philosophical, but to me it seems better to
refactor when possible.

>
> One question about granularity: tcmalloc's allocation granularity is 8,
so
> by using 64- or 32- byte granules you lose some of the precision.
>

It depends what you mean by precision. For a single allocation, I end up
accumulating the access counts across all corresponding shadows (i.e.
if
>64 bytes). Although I also added in a measure of utilization, which is at
the less precise 64-byte granularity. I'm aligning everything to 32 bytes,
which with the 32-byte header means there is no aliasing of shadows between
different allocations. That being said...


> How important is that?
> Another option is to use 8- or 16-byte granularity and 4-byte (saturated?)
> counters.
>

We've discussed using smaller granularities with saturating math, along
with sampling to reduce the saturation.


> Lots of options to play with -- which is why my recommendation is to start
> with a callback-based instrumentation (just use tsan compiler pass for
> that),
> figure out exactly what you need and then either implement the one best
> instrumentation inline, or force the callbacks to be inlined with
> (Thin)LTO.
>

I'm not convinced yet that using callbacks makes this difficult to play
with, as it can be configured pretty easily from options, and the inline
instrumentation part has been pretty trivial thus far. We'd also have to
build in the counter update consolidation in either case (either with less
counter updates or with fewer/aggregated callbacks).


> OTOH, with smaller granularity you lose most of the "counter update
> consolidation" optimization.
>

Some of it (at least where we would consolidate updates to different fields
using alignment guarantees). But we'd still be able to aggregate across
potentially aliasing calls, including out of loop bodies, where the updates
are to the same field.


>
> Eagerly looking forward to using this profiler!
>

Thanks!
Teresa


>
> --kcc
>
>>
>> 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 |
>>
>

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