# ProtoZero design document ProtoZero is a zero-copy zero-alloc zero-syscall protobuf serialization libary purposefully built for Perfetto's tracing use cases. ## Motivations ProtoZero has been designed and optimized for proto serialization, which is used by all Perfetto tracing paths. Deserialization was introduced only at a later stage of the project and is mainly used by offline tools (e.g., [TraceProcessor](/docs/analysis/trace-processor.md). The _zero-copy zero-alloc zero-syscall_ statement applies only to the serialization code. Perfetto makes extensive use of protobuf in tracing fast-paths. Every trace event in Perfetto is a proto (see [TracePacket](/docs/reference/trace-packet-proto.autogen) reference). This allows events to be strongly typed and makes it easier for the team to maintain backwards compatibility using a language that is understood across the board. Tracing fast-paths need to have very little overhead, because instrumentation points are sprinkled all over the codebase of projects like Android and Chrome and are performance-critical. Overhead here is not just defined as CPU time (or instructions retired) it takes to execute the instrumentation point. A big source of overhead in a tracing system is represented by the working set of the instrumentation points, specifically extra I-cache and D-cache misses which would slow down the non-tracing code _after_ the tracing instrumentation point. The major design departures of ProtoZero from canonical C++ protobuf libraries like [libprotobuf](https://github.com/google/protobuf) are: * Treating serialization and deserialization as different use-cases served by different code. * Optimizing for binary size and working-set-size on the serialization paths. * Ignoring most of the error checking and long-tail features of protobuf (repeated vs optional, full type checks). * ProtoZero is not designed as general-purpose protobuf de/serialization and is heavily customized to maintain the tracing writing code minimal and allow the compiler to see through the architectural layers. * Code generated by ProtoZero needs to be hermetic. When building the amalgamated [Tracing SDK](/docs/instrumentation/tracing-sdk.md), the all perfetto tracing sources need to not have any dependency on any other libraries other than the C++ standard library and C library. ## Usage At the build-system level, ProtoZero is extremely similar to the conventional libprotobuf library. The ProtoZero `.proto -> .pbzero.{cc,h}` compiler is based on top of the libprotobuf parser and compiler infrastructure. ProtoZero is as a `protoc` compiler plugin. ProtoZero has a build-time-only dependency on libprotobuf (the plugin depends on libprotobuf's parser and compiler). The `.pbzero.{cc,h}` code generated by it, however, has no runtime dependency (not even header-only dependencies) on libprotobuf. In order to generate ProtoZero stubs from proto you need to: 1. Build the ProtoZero compiler plugin, which lives in [src/protozero/protoc_plugin/](/src/protozero/protoc_plugin/). ```bash tools/ninja -C out/default protozero_plugin protoc ``` 2. Invoke the libprotobuf `protoc` compiler passing the `protozero_plugin`: ```bash out/default/protoc \ --plugin=protoc-gen-plugin=out/default/protozero_plugin \ --plugin_out=wrapper_namespace=pbzero:/tmp/ \ test_msg.proto ``` This generates `/tmp/test_msg.pbzero.{cc,h}`. NOTE: The .cc file is always empty. ProtoZero-generated code is header only. The .cc file is emitted only because some build systems' rules assume that protobuf codegens generate both a .cc and a .h file. ## Proto serialization The quickest way to undestand ProtoZero design principles is to start from a small example and compare the generated code between libprotobuf and ProtoZero. ```protobuf syntax = "proto2"; message TestMsg { optional string str_val = 1; optional int32 int_val = 2; repeated TestMsg nested = 3; } ``` #### libprotobuf approach The libprotobuf approach is to generate a C++ class that has one member for each proto field, with dedicated serialization and de-serialization methods. ```bash out/default/protoc --cpp_out=. test_msg.proto ``` generates test_msg.pb.{cc,h}. With many degrees of simplification, it looks as follows: ```c++ // This class is generated by the standard protoc compiler in the .pb.h source. class TestMsg : public protobuf::MessageLite { private: int32 int_val_; ArenaStringPtr str_val_; RepeatedPtrField nested_; // Effectively a vector public: const std::string& str_val() const; void set_str_val(const std::string& value); bool has_int_val() const; int32_t int_val() const; void set_int_val(int32_t value); ::TestMsg* add_nested(); ::TestMsg* mutable_nested(int index); const TestMsg& nested(int index); std::string SerializeAsString(); bool ParseFromString(const std::string&); } ``` The main characteristic of these stubs are: * Code generated from .proto messages can be used in the codebase as general purpose objects, without ever using the `SerializeAs*()` or `ParseFrom*()` methods (although anecdotal evidence suggests that most project use these proto-generated classes only at the de/serialization endpoints). * The end-to-end journey of serializing a proto involves two steps: 1. Setting the individual int / string / vector fields of the generated class. 2. Doing a serialization pass over these fields. In turn this has side-effects on the code generated. STL copy/assignment operators for strings and vectors are non-trivial because, for instance, they need to deal with dynamic memory resizing. #### ProtoZero approach ```c++ // This class is generated by the ProtoZero plugin in the .pbzero.h source. class TestMsg : public protozero::Message { public: void set_str_val(const std::string& value) { AppendBytes(/*field_id=*/1, value.data(), value.size()); } void set_str_val(const char* data, size_t size) { AppendBytes(/*field_id=*/1, data, size); } void set_int_val(int32_t value) { AppendVarInt(/*field_id=*/2, value); } TestMsg* add_nested() { return BeginNestedMessage(/*field_id=*/3); } } ``` The ProtoZero-generated stubs are append-only. As the `set_*`, `add_*` methods are invoked, the passed arguments are directly serialized into the target buffer. This introduces some limitations: * Readback is not possible: these classes cannot be used as C++ struct replacements. * No error-checking is performed: nothing prevents a non-repeated field to be emitted twice in the serialized proto if the caller accidentally calls a `set_*()` method twice. Basic type checks are still performed at compile-time though. * Nested fields must be filled in a stack fashion and cannot be written interleaved. Once a nested message is started, its fields must be set before going back setting the fields of the parent message. This turns out to not be a problem for most tracing use-cases. This has a number of advantages: * The classes generated by ProtoZero don't add any extra state on top of the base class they derive (`protozero::Message`). They define only inline setter methods that call base-class serialization methods. Compilers can see through all the inline expansions of these methods. * As a consequence of that, the binary cost of ProtoZero is independent of the number of protobuf messages defined and their fields, and depends only on the number of `set_*`/`add_*` calls. This (i.e. binary cost of non-used proto messages and fields) anecdotally has been a big issue with libprotobuf. * The serialization methods don't involve any copy or dynamic allocation. The inline expansion calls directly into the corresponding `AppendVarInt()` / `AppendString()` methods of `protozero::Message`. * This allows to directly serialize trace events into the [tracing shared memory buffers](/docs/concepts/buffers.md), even if they are not contiguous. ### Scattered buffer writing A key part of the ProtoZero design is supporting direct serialization on non-globally-contiguous sequences of contiguous memory regions. This happens by decoupling `protozero::Message`, the base class for all the generated classes, from the `protozero::ScatteredStreamWriter`. The problem it solves is the following: ProtoZero is based on direct serialization into shared memory buffers chunks. These chunks are 4KB - 32KB in most cases. At the same time, there is no limit in how much data the caller will try to write into an individual message, a trace event can be up to 256 MiB big.  #### Fast-path At all times the underlying `ScatteredStreamWriter` knows what are the bounds of the current buffer. All write operations are bound checked and hit a slow-path when crossing the buffer boundary. Most write operations can be completed within the current buffer boundaries. In that case, the cost of a `set_*` operation is in essence a `memcpy()` with the extra overhead of var-int encoding for protobuf preambles and length-delimited fields. #### Slow-path When crossing the boundary, the slow-path asks the `ScatteredStreamWriter::Delegate` for a new buffer. The implementation of `GetNewBuffer()` is up to the client. In tracing use-cases, that call will acquire a new thread-local chunk from the tracing shared memory buffer. Other heap-based implementations are possible. For instance, the ProtoZero sources provide a helper class `HeapBuffered`, mainly used in tests (see [scattered_heap_buffer.h](/include/perfetto/protozero/scattered_heap_buffer.h)), which allocates a new heap buffer when crossing the boundaries of the current one. Consider the following example: ```c++ TestMsg outer_msg; for (int i = 0; i < 1000; i++) { TestMsg* nested = outer_msg.add_nested(); nested->set_int_val(42); } ``` At some point one of the `set_int_val()` calls will hit the slow-path and acquire a new buffer. The overall idea is having a serialization mechanism that is extremely lightweight most of the times and that requires some extra function calls when buffer boundary, so that their cost gets amortized across all trace events. In the context of the overall Perfetto tracing use case, the slow-path involves grabbing a process-local mutex and finding the next free chunk in the shared memory buffer. Hence writes are lock-free as long as they happen within the thread-local chunk and require a critical section to acquire a new chunk once every 4KB-32KB (depending on the trace configuration). The assumption is that the likeliness that two threads will cross the chunk boundary and call `GetNewBuffer()` at the same time is extremely low and hence the critical section is un-contended most of the times. ```mermaid sequenceDiagram participant C as Call site participant M as Message participant SSR as ScatteredStreamWriter participant DEL as Buffer Delegate C->>M: set_int_val(...) activate C M->>SSR: AppendVarInt(...) deactivate C Note over C,SSR: A typical write on the fast-path C->>M: set_str_val(...) activate C M->>SSR: AppendString(...) SSR->>DEL: GetNewBuffer(...) deactivate C Note over C,DEL: A write on the slow-path when crossing 4KB - 32KB chunks. ``` ### Deferred patching Nested messages in the protobuf binary encoding are prefixed with their varint-encoded size. Consider the following: ```c++ TestMsg* nested = outer_msg.add_nested(); nested->set_int_val(42); nested->set_str_val("foo"); ``` The canonical encoding of this protobuf message, using libprotobuf, would be: ```bash 1a 07 0a 03 66 6f 6f 10 2a ^-+-^ ^-----+------^ ^-+-^ | | | | | +--> Field ID: 2 [int_val], value = 42. | | | +------> Field ID: 1 [str_val], len = 3, value = "foo" (66 6f 6f). | +------> Field ID: 3 [nested], length: 7 # !!! ``` The second byte in this sequence (07) is problematic for direct encoding. At the point where `outer_msg.add_nested()` is called, we can't possibly know upfront what the overall size of the nested message will be (in this case, 5 + 2 = 7). The way we get around this in ProtoZero is by reserving four bytes for the _size_ of each nested message and back-filling them once the message is finalized (or when we try to set a field in one of the parent messages). We do this by encoding the size of the message using redundant varint encoding, in this case: `87 80 80 00` instead of `07`. At the C++ level, the `protozero::Message` class holds a pointer to its `size` field, which typically points to the beginning of the message, where the four bytes are reserved, and back-fills it in the `Message::Finalize()` pass. This works fine for cases where the entire message lies in one contiguous buffer but opens a further challenge: a message can be several MBs big. Looking at this from the overall tracing perspective, the shared memory buffer chunk that holds the beginning of a message can be long gone (i.e. committed in the central service buffer) by the time we get to the end. In order to support this use case, at the tracing code level (outside of ProtoZero), when a message crosses the buffer boundary, its `size` field gets redirected to a temporary patch buffer (see [patch_list.h](/src/tracing/core/patch_list.h)). This patch buffer is then sent out-of-band, piggybacking over the next commit IPC (see [Tracing Protocol ABI](/docs/design-docs/api-and-abi.md#tracing-protocol-abi)) ### Performance characteristics NOTE: For the full code of the benchmark see `/src/protozero/test/protozero_benchmark.cc` We consider two scenarios: writing a simple event and a nested event #### Simple event Consists of filling a flat proto message with of 4 integers (2 x 32-bit, 2 x 64-bit) and a 32 bytes string, as follows: ```c++ void FillMessage_Simple(T* msg) { msg->set_field_int32(...); msg->set_field_uint32(...); msg->set_field_int64(...); msg->set_field_uint64(...); msg->set_field_string(...); } ``` #### Nested event Consists of filling a similar message which is recursively nested 3 levels deep: ```c++ void FillMessage_Nested(T* msg, int depth = 0) { FillMessage_Simple(msg); if (depth < 3) { auto* child = msg->add_field_nested(); FillMessage_Nested(child, depth + 1); } } ``` #### Comparison terms We compare, for the same message type, the performance of ProtoZero, libprotobuf and a speed-of-light serializer. The speed-of-light serializer is a very simple C++ class that just appends data into a linear buffer making all sorts of favourable assumptions. It does not use any binary-stable encoding, it does not perform bound checking, all writes are 64-bit aligned, it doesn't deal with any thread-safety. ```c++ struct SOLMsg { template void Append(T x) { // The memcpy will be elided by the compiler, which will emit just a // 64-bit aligned mov instruction. memcpy(reinterpret_cast(ptr_), &x, sizeof(x)); ptr_ += sizeof(x); } void set_field_int32(int32_t x) { Append(x); } void set_field_uint32(uint32_t x) { Append(x); } void set_field_int64(int64_t x) { Append(x); } void set_field_uint64(uint64_t x) { Append(x); } void set_field_string(const char* str) { ptr_ = strcpy(ptr_, str); } alignas(uint64_t) char storage_[sizeof(g_fake_input_simple) + 8]; char* ptr_ = &storage_[0]; }; ``` The speed-of-light serializer serves as a reference for _how fast a serializer could be if argument marshalling and bound checking were zero cost._ #### Benchmark results ##### Google Pixel 3 - aarch64 ```bash $ cat out/droid_arm64/args.gn target_os = "android" is_clang = true is_debug = false target_cpu = "arm64" $ ninja -C out/droid_arm64/ perfetto_benchmarks && \ adb push --sync out/droid_arm64/perfetto_benchmarks /data/local/tmp/perfetto_benchmarks && \ adb shell '/data/local/tmp/perfetto_benchmarks --benchmark_filter=BM_Proto*' ------------------------------------------------------------------------ Benchmark Time CPU Iterations ------------------------------------------------------------------------ BM_Protozero_Simple_Libprotobuf 402 ns 398 ns 1732807 BM_Protozero_Simple_Protozero 242 ns 239 ns 2929528 BM_Protozero_Simple_SpeedOfLight 118 ns 117 ns 6101381 BM_Protozero_Nested_Libprotobuf 1810 ns 1800 ns 390468 BM_Protozero_Nested_Protozero 780 ns 773 ns 901369 BM_Protozero_Nested_SpeedOfLight 138 ns 136 ns 5147958 ``` ##### HP Z920 workstation (Intel Xeon E5-2690 v4) running Linux ```bash $ cat out/linux_clang_release/args.gn is_clang = true is_debug = false $ ninja -C out/linux_clang_release/ perfetto_benchmarks && \ out/linux_clang_release/perfetto_benchmarks --benchmark_filter=BM_Proto* ------------------------------------------------------------------------ Benchmark Time CPU Iterations ------------------------------------------------------------------------ BM_Protozero_Simple_Libprotobuf 428 ns 428 ns 1624801 BM_Protozero_Simple_Protozero 261 ns 261 ns 2715544 BM_Protozero_Simple_SpeedOfLight 111 ns 111 ns 6297387 BM_Protozero_Nested_Libprotobuf 1625 ns 1625 ns 436411 BM_Protozero_Nested_Protozero 843 ns 843 ns 849302 BM_Protozero_Nested_SpeedOfLight 140 ns 140 ns 5012910 ```