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References: <20210217154844.12392-1-david@redhat.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to
 prefault/prealloc memory
Message-ID: <4bb9071b-e6c1-a732-0ed6-46aff0eaa70c@redhat.com>
Date:   Wed, 24 Feb 2021 15:25:50 +0100
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> +		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