Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory

David Hildenbrand
2021-02-24T15:42:25Z

> + tmp_end = min_t(unsigned long, end, vma->vm_end);
> + pages = populate_vma_page_range(vma, start, tmp_end, &locked);
> + if (!locked) {
> + mmap_read_lock(mm);
> + *prev = NULL;
> + vma = NULL;

^ locked = 1; is missing here.

--- Simple benchmark ---

I implemented MADV_POPULATE_READ and MADV_POPULATE_WRITE and performed
some simple measurements to simulate memory preallocation with empty files:

1) mmap a 2 MiB/128 MiB/4 GiB region (anonymous, memfd, memfd hugetlb)
2) Discard all memory using fallocate/madvise
3) Prefault memory using different approaches and measure the time this
takes.

I repeat 2)+3) 10 times and compute the average. I only use a single thread.

Read: Read from each page a byte.
Write: Write one byte of each page (0).
Read/Write: Read one byte and write the value back for each page
POPULATE: MADV_POPULATE (this patch)
POPULATE_READ: MADV_POPULATE_READ
POPULATE_WRITE: MADV_POPULATE_WRITE

--- Benchmark results ---

Measuring 10 iterations each:
==================================================
2 MiB MAP_PRIVATE:
**************************************************
Anonymous : Read : 0.159 ms
Anonymous : Write : 0.244 ms
Anonymous : Read+Write : 0.383 ms
Anonymous : POPULATE : 0.167 ms
Anonymous : POPULATE_READ : 0.064 ms
Anonymous : POPULATE_WRITE : 0.165 ms
Memfd 4 KiB : Read : 0.401 ms
Memfd 4 KiB : Write : 0.056 ms
Memfd 4 KiB : Read+Write : 0.075 ms
Memfd 4 KiB : POPULATE : 0.057 ms
Memfd 4 KiB : POPULATE_READ : 0.337 ms
Memfd 4 KiB : POPULATE_WRITE : 0.056 ms
Memfd 2 MiB : Read : 0.041 ms
Memfd 2 MiB : Write : 0.030 ms
Memfd 2 MiB : Read+Write : 0.031 ms
Memfd 2 MiB : POPULATE : 0.031 ms
Memfd 2 MiB : POPULATE_READ : 0.031 ms
Memfd 2 MiB : POPULATE_WRITE : 0.031 ms
**************************************************
2 MiB MAP_SHARED:
**************************************************
Anonymous : Read : 0.071 ms
Anonymous : Write : 0.181 ms
Anonymous : Read+Write : 0.081 ms
Anonymous : POPULATE : 0.069 ms
Anonymous : POPULATE_READ : 0.069 ms
Anonymous : POPULATE_WRITE : 0.115 ms
Memfd 4 KiB : Read : 0.401 ms
Memfd 4 KiB : Write : 0.351 ms
Memfd 4 KiB : Read+Write : 0.414 ms
Memfd 4 KiB : POPULATE : 0.338 ms
Memfd 4 KiB : POPULATE_READ : 0.339 ms
Memfd 4 KiB : POPULATE_WRITE : 0.279 ms
Memfd 2 MiB : Read : 0.031 ms
Memfd 2 MiB : Write : 0.031 ms
Memfd 2 MiB : Read+Write : 0.031 ms
Memfd 2 MiB : POPULATE : 0.031 ms
Memfd 2 MiB : POPULATE_READ : 0.031 ms
Memfd 2 MiB : POPULATE_WRITE : 0.031 ms
**************************************************
128 MiB MAP_PRIVATE:
**************************************************
Anonymous : Read : 7.517 ms
Anonymous : Write : 22.503 ms
Anonymous : Read+Write : 33.186 ms
Anonymous : POPULATE : 18.381 ms
Anonymous : POPULATE_READ : 3.952 ms
Anonymous : POPULATE_WRITE : 18.354 ms
Memfd 4 KiB : Read : 34.300 ms
Memfd 4 KiB : Write : 4.659 ms
Memfd 4 KiB : Read+Write : 6.531 ms
Memfd 4 KiB : POPULATE : 5.219 ms
Memfd 4 KiB : POPULATE_READ : 29.744 ms
Memfd 4 KiB : POPULATE_WRITE : 5.244 ms
Memfd 2 MiB : Read : 10.228 ms
Memfd 2 MiB : Write : 10.130 ms
Memfd 2 MiB : Read+Write : 10.190 ms
Memfd 2 MiB : POPULATE : 10.007 ms
Memfd 2 MiB : POPULATE_READ : 10.008 ms
Memfd 2 MiB : POPULATE_WRITE : 10.010 ms
**************************************************
128 MiB MAP_SHARED:
**************************************************
Anonymous : Read : 7.295 ms
Anonymous : Write : 15.234 ms
Anonymous : Read+Write : 7.460 ms
Anonymous : POPULATE : 5.196 ms
Anonymous : POPULATE_READ : 5.190 ms
Anonymous : POPULATE_WRITE : 8.245 ms
Memfd 4 KiB : Read : 34.412 ms
Memfd 4 KiB : Write : 30.586 ms
Memfd 4 KiB : Read+Write : 35.157 ms
Memfd 4 KiB : POPULATE : 29.643 ms
Memfd 4 KiB : POPULATE_READ : 29.691 ms
Memfd 4 KiB : POPULATE_WRITE : 25.790 ms
Memfd 2 MiB : Read : 10.210 ms
Memfd 2 MiB : Write : 10.074 ms
Memfd 2 MiB : Read+Write : 10.068 ms
Memfd 2 MiB : POPULATE : 10.034 ms
Memfd 2 MiB : POPULATE_READ : 10.037 ms
Memfd 2 MiB : POPULATE_WRITE : 10.031 ms
**************************************************
4096 MiB MAP_PRIVATE:
**************************************************
Anonymous : Read : 240.947 ms
Anonymous : Write : 712.941 ms
Anonymous : Read+Write : 1027.636 ms
Anonymous : POPULATE : 571.816 ms
Anonymous : POPULATE_READ : 120.215 ms
Anonymous : POPULATE_WRITE : 570.750 ms
Memfd 4 KiB : Read : 1054.739 ms
Memfd 4 KiB : Write : 145.534 ms
Memfd 4 KiB : Read+Write : 202.275 ms
Memfd 4 KiB : POPULATE : 162.597 ms
Memfd 4 KiB : POPULATE_READ : 914.747 ms
Memfd 4 KiB : POPULATE_WRITE : 161.281 ms
Memfd 2 MiB : Read : 351.818 ms
Memfd 2 MiB : Write : 352.357 ms
Memfd 2 MiB : Read+Write : 352.762 ms
Memfd 2 MiB : POPULATE : 351.471 ms
Memfd 2 MiB : POPULATE_READ : 351.553 ms
Memfd 2 MiB : POPULATE_WRITE : 351.931 ms
**************************************************
4096 MiB MAP_SHARED:
**************************************************
Anonymous : Read : 229.338 ms
Anonymous : Write : 478.964 ms
Anonymous : Read+Write : 234.546 ms
Anonymous : POPULATE : 161.635 ms
Anonymous : POPULATE_READ : 160.943 ms
Anonymous : POPULATE_WRITE : 252.686 ms
Memfd 4 KiB : Read : 1052.828 ms
Memfd 4 KiB : Write : 929.237 ms
Memfd 4 KiB : Read+Write : 1074.494 ms
Memfd 4 KiB : POPULATE : 915.663 ms
Memfd 4 KiB : POPULATE_READ : 915.001 ms
Memfd 4 KiB : POPULATE_WRITE : 787.388 ms
Memfd 2 MiB : Read : 353.580 ms
Memfd 2 MiB : Write : 353.197 ms
Memfd 2 MiB : Read+Write : 353.172 ms
Memfd 2 MiB : POPULATE : 353.686 ms
Memfd 2 MiB : POPULATE_READ : 353.465 ms
Memfd 2 MiB : POPULATE_WRITE : 352.776 ms
**************************************************

--- Discussion ---

1) With huge pages, the performance benefit is negligible with the sizes
I tried, because there are little actual page faults. Most time is spent
zeroing huge pages I guess. It will take quite a lot of memory to pay off.

2) In all 4k cases, the POPULATE_READ/POPULATE_WRITE variants are faster
than manually reading or writing from user space.

What sticks out a bit is:

3) For MAP_SHARED on anonymous memory, it is fastest to first read and
then write memory. It's slightly faster than POPULATE_WRITE and quite a
lot faster than a simple write - what?!. It's even faster than
POPULATE_WRITE - what?! I assume with the read access we prepare a fresh
zero page and with the write access we only have to change PTE access
rights. But why is this faster than writing directly?

4) POPULATE_WRITE with MAP_SHARED "Memfd 4 KiB" is faster than
POPULATE_READ - it's the fastest way to preallocate that memory.
Similarly, ordinary writes are faster than ordinary reads.

I did not try with actual files, files that already have a content, or
with multiple thread yet. Also, I did not try on a subset of a mmap yet
- for simplicity I populate the whole mapping.

--
Thanks,

David / dhildenb