[Bug libstdc++/96942] std::pmr::monotonic_buffer_resource causes CPU cache misses

[dmitriy.ovdienko at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96942

--- Comment #6 from Dmitriy Ovdienko <dmitriy.ovdienko at gmail dot com> ---
> looking at cache-misses counter does not make sense here

Well, if you compare Rust and C++, cache-misses CPU counter differs
dramatically... and page-faults too... while amount of instructions is the
same.

Page-faults btw, can significantly affect performance too. It could happen that
that is the reason.

I've put all numbers into one table for convenience:


| CPU counter      | PMR            | Malloc         | Rust           |
|------------------|----------------|----------------|----------------|
| cache-references |     45,104,136 |     40,713,525 |     29,268,774 |
| cache-misses     |     24,448,475 |     14,147,648 |     12,147,041 |
| cycles           | 19,904,251,283 | 14,823,743,812 | 24,539,557,585 |
| instructions     | 30,462,013,065 | 22,306,442,507 | 31,784,741,964 |
| branches         |  4,834,392,341 |  4,331,968,591 |  4,829,547,556 |
| faults           |        234,796 |         60,227 |         68,023 |
| migrations       |              2 |              6 |              8 |

> The main gotcha here is m_b_r does not allocate on construction, but rather allocates 2x of the preallocation size on first call to 'allocate'

In the two previous posts I've attached a code that does not create any thread
and allocates/deallocates memory in the loop. So, both samples have the same
behaviour.