niyue commented on pull request #11588:
URL: https://github.com/apache/arrow/pull/11588#issuecomment-961487813
@westonpace
> in MemoryMappedFile::ReadAt we call
::arrow::internal::MemoryAdviseWillNeed on the entire range accessed
Thanks for the pointer, and I found this `MemoryAdviseWillNeed` is called
inside `MemoryMappedFile::ReadAt` and is probably causing the problem I ran
into.
```
RETURN_NOT_OK(::arrow::internal::MemoryAdviseWillNeed(
{{memory_map_->data() + position, static_cast<size_t>(nbytes)}}));
```
I debugged the program previously and confirmed `MemoryMappedFile::ReadAt`
didn't prefetch the array data after calling it, and the real IO only happens
when the user program accesses the array value, so I thought mmap worked but OS
didn't work as expected. But I didn't think about that it advises OS to
prefetch this range and made OS to behave differently. Let me do some more
investigation to see how it works. Thanks so much.
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