On 08/10/2017 03:00 PM, Eric Biggers wrote:
On Thu, Aug 10, 2017 at 01:41:21PM -0400, Chris Mason wrote:
On 08/10/2017 04:30 AM, Eric Biggers wrote:
On Wed, Aug 09, 2017 at 07:35:53PM -0700, Nick Terrell wrote:


The memory reported is the amount of memory the compressor requests.

| Method   | Size (B) | Time (s) | Ratio | MB/s    | Adj MB/s | Mem (MB) |
|----------|----------|----------|-------|---------|----------|----------|
| none     | 11988480 |    0.100 |     1 | 2119.88 |        - |        - |
| zstd -1  | 73645762 |    1.044 | 2.878 |  203.05 |   224.56 |     1.23 |
| zstd -3  | 66988878 |    1.761 | 3.165 |  120.38 |   127.63 |     2.47 |
| zstd -5  | 65001259 |    2.563 | 3.261 |   82.71 |    86.07 |     2.86 |
| zstd -10 | 60165346 |   13.242 | 3.523 |   16.01 |    16.13 |    13.22 |
| zstd -15 | 58009756 |   47.601 | 3.654 |    4.45 |     4.46 |    21.61 |
| zstd -19 | 54014593 |  102.835 | 3.925 |    2.06 |     2.06 |    60.15 |
| zlib -1  | 77260026 |    2.895 | 2.744 |   73.23 |    75.85 |     0.27 |
| zlib -3  | 72972206 |    4.116 | 2.905 |   51.50 |    52.79 |     0.27 |
| zlib -6  | 68190360 |    9.633 | 3.109 |   22.01 |    22.24 |     0.27 |
| zlib -9  | 67613382 |   22.554 | 3.135 |    9.40 |     9.44 |     0.27 |


Theses benchmarks are misleading because they compress the whole file as a
single stream without resetting the dictionary, which isn't how data will
typically be compressed in kernel mode.  With filesystem compression the data
has to be divided into small chunks that can each be decompressed independently.
That eliminates one of the primary advantages of Zstandard (support for large
dictionary sizes).

I did btrfs benchmarks of kernel trees and other normal data sets as
well.  The numbers were in line with what Nick is posting here.
zstd is a big win over both lzo and zlib from a btrfs point of view.

It's true Nick's patches only support a single compression level in
btrfs, but that's because btrfs doesn't have a way to pass in the
compression ratio.  It could easily be a mount option, it was just
outside the scope of Nick's initial work.


I am not surprised --- Zstandard is closer to the state of the art, both
format-wise and implementation-wise, than the other choices in BTRFS.  My point
is that benchmarks need to account for how much data is compressed at a time.
This is a common mistake when comparing different compression algorithms; the
algorithm name and compression level do not tell the whole story.  The
dictionary size is extremely significant.  No one is going to compress or
decompress a 200 MB file as a single stream in kernel mode, so it does not make
sense to justify adding Zstandard *to the kernel* based on such a benchmark.  It
is going to be divided into chunks.  How big are the chunks in BTRFS?  I thought
that it compressed only one page (4 KiB) at a time, but I hope that has been, or
is being, improved; 32 KiB - 128 KiB should be a better amount.  (And if the
amount of data compressed at a time happens to be different between the
different algorithms, note that BTRFS benchmarks are likely to be measuring that
as much as the algorithms themselves.)

Btrfs hooks the compression code into the delayed allocation mechanism we use to gather large extents for COW. So if you write 100MB to a file, we'll have 100MB to compress at a time (within the limits of the amount of pages we allow to collect before forcing it down).

But we want to balance how much memory you might need to uncompress during random reads. So we have an artificial limit of 128KB that we send at a time to the compression code. It's easy to change this, it's just a tradeoff made to limit the cost of reading small bits.

It's the same for zlib,lzo and the new zstd patch.

-chris

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