pitrou commented on a change in pull request #12116:
URL: https://github.com/apache/arrow/pull/12116#discussion_r806152099
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File path: cpp/src/arrow/memory_pool.cc
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@@ -603,14 +643,109 @@ class BaseMemoryPoolImpl : public MemoryPool {
stats_.UpdateAllocatedBytes(-size);
}
- void ReleaseUnused() override { Allocator::ReleaseUnused(); }
+ protected:
+ virtual Status AllocateImmutableZeros(int64_t size, uint8_t** out) {
+#ifdef USE_MMAP_FOR_IMMUTABLE_ZEROS
+ if (size > 0) {
+ *out = static_cast<uint8_t*>(mmap(
+ nullptr, size, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS |
MAP_NORESERVE, -1, 0));
+ if (*out == MAP_FAILED) {
+ auto err = errno;
+ return Status::OutOfMemory("Failed to allocate zero buffer of size ",
size, ": ",
+ strerror(err));
+ }
+ return Status::OK();
+ }
+#endif
+ RETURN_NOT_OK(Allocate(size, out));
+ std::memset(*out, 0, size);
+ return Status::OK();
+ }
+
+ void FreeImmutableZeros(uint8_t* buffer, int64_t size) override {
+#ifdef USE_MMAP_FOR_IMMUTABLE_ZEROS
+ if (size > 0) {
+ munmap(buffer, size);
+ return;
+ }
+#endif
+ Free(buffer, size);
+ }
+
+ public:
+ Result<std::shared_ptr<Buffer>> GetImmutableZeros(int64_t size) override {
+ // Thread-safely get the current largest buffer of zeros.
Review comment:
> * I'm not sure if it's even needed, though: it isn't if you can safely
make a copy of a `shared_ptr` while another thread may be updating the
contained pointer.
You can safely update the object _pointed to_ (well, except that accessing
that object from different threads then becomes racy). However, changing the
_pointer_ requires use of dedicated atomic access functions:
https://en.cppreference.com/w/cpp/memory/shared_ptr/atomic
I guess this means you should favour changing the pointer, since all
accesses will be confined in this utility function.
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