Description:
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It may happen that memory is allocated and freed in such a way that the allocation mmaps a new ZMM chunk and the deallocation immediately unmaps it again.

The attached script demonstrates this issues. It will execute quickly up to a certain point (for me on master i=94) and then hang for a while. strace shows that during this time a sequence of mmap, madvise and munmap is repeated many times:

mmap(NULL, 2097152, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f3927400000
madvise(0x7f3927400000, 2097152, MADV_HUGEPAGE) = 0
munmap(0x7f3927400000, 2097152)         = 0

This issue may be fixed by ensuring we always cache at least one chunk.


Test script:
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<?php

$bigArray = range(0, 256 * 1024 - 1);

var_dump(memory_get_usage(true) / (1024*1024));
$array = [];
for ($i = 0; $i < 1024; $i++) {
    echo "$i\n";
    $array[] = range(0, 1023);
    for ($j = 0; $j < 16 * 1024; $j++) {
        $x = range(0, 1023);
        unset($x);
    }
}