Re: [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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