High memory usage when writing files to mount

[sb10]

There is high memory usage when writing files to S3 via muxfys mount.

make -f buildscripts/lint.makefile test
make -f buildscripts/lint.makefile race
go test -p 1 -tags netgo --count 1 ./... -v
CGO_ENABLED=1 go test -tags netgo . -v -race -run TestS3L

dd if=/dev/urandom of=/tmp/1G.file bs=1048576 count=1000

s3cmd rm -r s3://sb10/irods_test

wr manager start
perl -e 'for(1..1000){$n++; print "echo $_ && touch s3/touch && cp -fRL /tmp/1G.file s3/\n"}' | wr add -i memleak -r 0 --disk 1 -m 5G -o 2 --mount_json '[{"Mount":"s3","Targets":[{"Path":"sb10/irods_test","Write":true}]}]'
top -u ubuntu
wr manager stop
s3cmd rm -r s3://sb10/irods_test

Using muxfys v4:
Steady at 8.2%. At least the leak is fixed, but I don't like that it uses this memory, presumably dependent on how much data was read or written. Should still look at if this can be fixed.

In a test, uploading a large file immediately jumps from 70MB usage to 728MB and then doesn't change, with allocations going from 641MB and increasing every few seconds by 2MB or so, but also sometimes dropping.

I'm using https://golang.org/pkg/io/#Pipe which is a "synchronous in-memory pipe ... The data is copied directly from the Write to the corresponding Read (or Reads); there is no internal buffering."

So not an obvious buffer problem. Could just be related to the amount of data that is sent in or read out in one go? See what happens with files smaller and larger than ~700MB...

I believe that less memory is used for a cached mount. Check that as well.