jvanstraten commented on a change in pull request #12116:
URL: https://github.com/apache/arrow/pull/12116#discussion_r806689894
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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:
> That is true. However, it shouldn't be a concern if we can ensure that
the pages don't actually allocate physical memory (or almost none of it, such
as `/dev/zero`).
I don't think we can in general, unfortunately. I would have no idea how to
do it on something as ubiquitous as Windows (or if it can be done at all), and
I'm sure that in general there are more exotic operating systems and
architectures that simply can't do it. Also, for 32-bit systems/builds (if
Arrow supports those) virtual memory is also in relatively short supply.
> However, changing the pointer requires use of dedicated atomic access
functions: [...]
Ah, great, those functions are exactly what I had missed!
I improved my allocation algorithm accordingly here: e628688
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