Document MIN_PAYLOAD_SIZE and mark-compact GC mark bits

This updates the documentation of the MIN_PAYLOAD_SIZE constant and adds
a new Note [Mark bits in mark-compact collector] explaning why the
mark-compact collector uses two bits per objet and why we need
MIN_PAYLOAD_SIZE.
21 jobs for master in 461 minutes and 50 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Lint
passed #160608
lint
lint-linters

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lint
lint-submods-branch

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lint
lint-testsuite

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lint
typecheck-testsuite

00:00:19

 
  Build
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x86_64-linux
hadrian-ghc-in-ghci

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x86_64-linux
release-x86_64-linux-deb9-dwarf

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x86_64-linux
validate-x86_64-linux-deb9-debug

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x86_64-linux
validate-x86_64-linux-deb9-hadrian

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  Full Build
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aarch64-linux allowed to fail
validate-aarch64-linux-deb9

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x86_64-linux
validate-i386-linux-deb9

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x86_64-darwin
validate-x86_64-darwin

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x86_64-linux
validate-x86_64-linux-deb9-integer-simple

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x86_64-linux
validate-x86_64-linux-deb9-unreg

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x86_64-linux
validate-x86_64-linux-fedora27

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validate-x86_64-windows

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validate-x86_64-windows-hadrian

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  Cleanup
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cleanup-darwin

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  Packaging
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doc-tarball

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  Testing
manual #160626
x86_64-linux allowed to fail manual
hackage
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perf-nofib

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  Deploy
failed #160628
x86_64-linux allowed to fail
pages

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Name Stage Failure
failed
doc-tarball Packaging
$ git checkout .gitmodules
$ git fetch https://gitlab.haskell.org/ghc/ghc-performance-notes.git refs/notes/perf:refs/notes/perf || true
From https://gitlab.haskell.org/ghc/ghc-performance-notes
c5c4d4be84..ff9d03fa63 refs/notes/perf -> refs/notes/perf
Auto packing the repository in background for optimum performance.
See "git help gc" for manual housekeeping.
$ if [ ! -f "$LINUX_BINDIST" ]; then # collapsed multi-line command
Error: ghc-x86_64-mingw32.tar.xz does not exist. Did the 64-bit Windows job fail?
ERROR: Job failed: exit code 1
failed
pages Deploy
$ git checkout .gitmodules
$ git fetch https://gitlab.haskell.org/ghc/ghc-performance-notes.git refs/notes/perf:refs/notes/perf || true
From https://gitlab.haskell.org/ghc/ghc-performance-notes
d91e7817b4..ff9d03fa63 refs/notes/perf -> refs/notes/perf
$ mkdir -p public/doc
$ tar -xf haddock.html.tar.xz -C public/doc
tar: haddock.html.tar.xz: Cannot open: No such file or directory
tar: Error is not recoverable: exiting now
ERROR: Job failed: exit code 1
failed
validate-x86_64-windows-hadrian Full Build
Runtime platform                                    arch=amd64 os=windows pid=1680 revision=1f513601 version=11.10.1
WARNING: ghc.tar.xz: no matching files
WARNING: junit.xml: no matching files
ERROR: No files to upload
Uploading artifacts...
Runtime platform
arch=amd64 os=windows pid=3588 revision=1f513601 version=11.10.1
WARNING: junit.xml: no matching files
ERROR: No files to upload
ERROR: Job failed: exit status 9
failed
validate-x86_64-windows Full Build
Runtime platform                                    arch=amd64 os=windows pid=3016 revision=1f513601 version=11.10.1
WARNING: ghc-*.tar.xz: no matching files
WARNING: junit.xml: no matching files
ERROR: No files to upload
Uploading artifacts...
Runtime platform
arch=amd64 os=windows pid=300 revision=1f513601 version=11.10.1
WARNING: junit.xml: no matching files
ERROR: No files to upload
ERROR: Job failed: exit status 9
failed
validate-aarch64-linux-deb9 Full Build
$ cp -Rf $HOME/.cabal cabal-cache
Uploading artifacts...
WARNING: ghc-*.tar.xz: no matching files
WARNING: junit.xml: no matching files
ERROR: No files to upload
Uploading artifacts...
WARNING: junit.xml: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1