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From:   David Hildenbrand <david@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>, Michal Hocko <mhocko@suse.com>,
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        Jason Gunthorpe <jgg@ziepe.ca>,
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        linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org,
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References: <20210217154844.12392-1-david@redhat.com>
 <4bb9071b-e6c1-a732-0ed6-46aff0eaa70c@redhat.com>
Organization: Red Hat GmbH
Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to
 prefault/prealloc memory
Message-ID: <e78dfe59-3552-91f1-e234-5f37d2600eb6@redhat.com>
Date:   Thu, 25 Feb 2021 09:41:07 +0100
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On 24.02.21 15:25, David Hildenbrand wrote:
>> +		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?

Okay, MADV_DONTNEED does not seem to really work on MAP_SHARED of 
anonymous memory. If I use a fresh mmap for each and every iteration the 
numbers make more sense:

**************************************************
4096 MiB MAP_SHARED:
**************************************************
Anonymous      : Read           :  1054.154 ms
Anonymous      : Write          :   924.572 ms
Anonymous      : Read+Write     :  1075.215 ms
Anonymous      : POPULATE       :   911.386 ms
Anonymous      : POPULATE_READ  :   909.392 ms
Anonymous      : POPULATE_WRITE :   793.143 ms


-- 
Thanks,

David / dhildenb