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SubjectRe: [PATCH v2] mm: implement write-behind policy for sequential file writes
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On 23/09/2019 17.52, Tejun Heo wrote:
> Hello, Konstantin.
>
> On Fri, Sep 20, 2019 at 10:39:33AM +0300, Konstantin Khlebnikov wrote:
>> With vm.dirty_write_behind 1 or 2 files are written even faster and
>
> Is the faster speed reproducible? I don't quite understand why this
> would be.

Writing to disk simply starts earlier.

>
>> during copying amount of dirty memory always stays around at 16MiB.
>
> The following is the test part of a slightly modified version of your
> test script which should run fine on any modern systems.
>
> for mode in 0 1; do
> if [ $mode == 0 ]; then
> prefix=''
> else
> prefix='systemd-run --user --scope -p MemoryMax=64M'
> fi
>
> echo COPY
> time $prefix cp -r dummy copy
>
> grep Dirty /proc/meminfo
>
> echo SYNC
> time sync
>
> rm -fr copy
> done
>
> and the result looks like the following.
>
> $ ./test-writebehind.sh
> SIZE
> 3.3G dummy
> COPY
>
> real 0m2.859s
> user 0m0.015s
> sys 0m2.843s
> Dirty: 3416780 kB
> SYNC
>
> real 0m34.008s
> user 0m0.000s
> sys 0m0.008s
> COPY
> Running scope as unit: run-r69dca5326a9a435d80e036435ff9e1da.scope
>
> real 0m32.267s
> user 0m0.032s
> sys 0m4.186s
> Dirty: 14304 kB
> SYNC
>
> real 0m1.783s
> user 0m0.000s
> sys 0m0.006s
>
> This is how we are solving the massive dirtier problem. It's easy,
> works pretty well and can easily be tailored to the specific
> requirements.
>
> Generic write-behind would definitely have other benefits and also a
> bunch of regression possibilities. I'm not trying to say that
> write-behind isn't a good idea but it'd be useful to consider that a
> good portion of the benefits can already be obtained fairly easily.
>

I'm afraid this could end badly if each simple task like file copying
will require own systemd job and container with manual tuning.

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