Fix hashtablez oversampling for SOO tables on repeated 0->1 element transitions Currently, when an absl::flat_hash_set or flat_hash_map with SOO (Single Object Optimization) transitions from 0 to 1 element, it queries should_sample_soo() to determine if it should be sampled by hashtablez. If it is NOT sampled, it remains an SOO table. If that element is subsequently erased, the table goes back to 0 elements. The next time it goes from 0 to 1, it queries should_sample_soo() again. Repeated insert/erase cycles on an SOO table effectively evaluate the sampling logic hundreds of times, making it extremely likely that the table eventually gets sampled and moved to the heap. This causes hashtablez to vastly overstate the memory usage of such maps in production. This CL fixes this by tracking whether an SOO table has already been evaluated for sampling, so it only evaluates the logic once per table object instance. Since SOO tables do not hash keys, they do not need the per-table generic seed value. This CL utilizes the lowest bit of the unused seed field within HashtableSize (accessed via data_ & 1) to act as a flag soo_has_tried_sampling. When an SOO table transitions from 0 elements to 1 element, this bit is checked. If it is false, should_sample_soo() is queried and the bit is set to true. set_full_soo() and set_empty_soo() have been modified to preserve the soo_has_tried_sampling bit across erase or clear operations. PiperOrigin-RevId: 881570273 Change-Id: Ia3b0eff8b3e137b0d162a207220a184923e3a2ca
The repository contains the Abseil C++ library code. Abseil is an open-source collection of C++ code (compliant to C++17) designed to augment the C++ standard library.
Abseil is an open-source collection of C++ library code designed to augment the C++ standard library. The Abseil library code is collected from Google's own C++ code base, has been extensively tested and used in production, and is the same code we depend on in our daily coding lives.
In some cases, Abseil provides pieces missing from the C++ standard; in others, Abseil provides alternatives to the standard for special needs we've found through usage in the Google code base. We denote those cases clearly within the library code we provide you.
Abseil is not meant to be a competitor to the standard library; we've just found that many of these utilities serve a purpose within our code base, and we now want to provide those resources to the C++ community as a whole.
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Abseil contains the following C++ library components:
base base library contains initialization code and other code which all other Abseil code depends on. Code within base may not depend on any other code (other than the C++ standard library).algorithm algorithm library contains additions to the C++ <algorithm> library and container-based versions of such algorithms.cleanup cleanup library contains the control-flow-construct-like type absl::Cleanup which is used for executing a callback on scope exit.container container library contains additional STL-style containers, including Abseil's unordered “Swiss table” containers.crc The crc library contains code for computing error-detecting cyclic redundancy checks on data.debugging debugging library contains code useful for enabling leak checks, and stacktrace and symbolization utilities.flags flags library contains code for handling command line flags for libraries and binaries built with Abseil.hash hash library contains the hashing framework and default hash functor implementations for hashable types in Abseil.log log library contains LOG and CHECK macros and facilities for writing logged messages out to disk, stderr, or user-extensible destinations.memory memory library contains memory management facilities that augment C++'s <memory> library.meta meta library contains type checks similar to those available in the C++ <type_traits> library.numeric numeric library contains 128-bit integer types as well as implementations of C++20's bitwise math functions.profiling profiling library contains utility code for profiling C++ entities. It is currently a private dependency of other Abseil libraries.random random library contains functions for generating pseudorandom values.status status library contains abstractions for error handling, specifically absl::Status and absl::StatusOr<T>.strings strings library contains a variety of strings routines and utilities.synchronization synchronization library contains concurrency primitives (Abseil's absl::Mutex class, an alternative to std::mutex) and a variety of synchronization abstractions.time time library contains abstractions for computing with absolute points in time, durations of time, and formatting and parsing time within time zones.types types library contains non-container utility types.utility utility library contains utility and helper code.Abseil recommends users “live-at-head” (update to the latest commit from the master branch as often as possible). However, we realize this philosophy doesn't work for every project, so we also provide Long Term Support Releases to which we backport fixes for severe bugs. See our release management document for more details.
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