convert ExportLocation to ShortByteString to avoid PINNED memory fragmentation

This adds the overhead of a copy whenever converting to/from ExportLocation and
ImportLocation.

borg: Some improvements to memory use when importing a lot of archives.
(It's still pretty bad.)

Sponsored-by: Mark Reidenbach on Patreon

CHANGELOG

@@ -15,6 +15,7 @@ git-annex (8.20210904) UNRELEASED; urgency=medium
     incrementally verified, when used on NTFS and perhaps other filesystems.
   * reinject: Fix crash when reinjecting a file from outside the repository.
     (Reversion in version 8.20210621)
+  * borg: Some improvements to memory use when importing a lot of archives.
 
  -- Joey Hess <id@joeyh.name>  Fri, 03 Sep 2021 12:02:55 -0400
 

Types/Export.hs

@@ -1,6 +1,6 @@
 {- git-annex export types
  -
- - Copyright 2017 Joey Hess <id@joeyh.name>
+ - Copyright 2017-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -21,23 +21,28 @@ import Git.FilePath
 import Utility.Split
 import Utility.FileSystemEncoding
 
+import Data.ByteString.Short as S
 import qualified System.FilePath.Posix as Posix
 import GHC.Generics
 import Control.DeepSeq
 
--- A location on a remote that a key can be exported to.
--- The RawFilePath will be relative to the top of the remote,
--- and uses unix-style path separators.
-newtype ExportLocation = ExportLocation RawFilePath
+-- A location such as a path on a remote, that a key can be exported to.
+-- The path is relative to the top of the remote, and uses unix-style
+-- path separators.
+--
+-- This uses a ShortByteString to avoid problems with ByteString getting
+-- PINNED in memory which caused memory fragmentation and excessive memory
+-- use.
+newtype ExportLocation = ExportLocation S.ShortByteString
 	deriving (Show, Eq, Generic)
 
 instance NFData ExportLocation
 
 mkExportLocation :: RawFilePath -> ExportLocation
-mkExportLocation = ExportLocation . toInternalGitPath
+mkExportLocation = ExportLocation . S.toShort . toInternalGitPath
 
 fromExportLocation :: ExportLocation -> RawFilePath
-fromExportLocation (ExportLocation f) = f
+fromExportLocation (ExportLocation f) = S.fromShort f
 
 newtype ExportDirectory = ExportDirectory RawFilePath
 	deriving (Show, Eq)
@@ -58,4 +63,4 @@ exportDirectories (ExportLocation f) =
 	subs ps (d:ds) = (d:ps) : subs (d:ps) ds
 
 	dirs = map Posix.dropTrailingPathSeparator $
-		dropFromEnd 1 $ Posix.splitPath $ decodeBS f
+		dropFromEnd 1 $ Posix.splitPath $ decodeBS $ S.fromShort f

doc/bugs/borg_special_remote_memory_usage_high_for_large_borg_repo/comment_3_de81876865308a0c707011bcdb0ba843._comment

@@ -0,0 +1,30 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 3"""
+ date="2021-10-05T17:39:29Z"
+ content="""
+A heap profile shows that the problem is an accumulation of PINNED memory.
+Both the memory used by borg list and by constructing the git tree.
+
+At least the borg list part seems very similar to the problem described here.
+<https://well-typed.com/blog/2020/08/memory-fragmentation/>
+The borg list gets read into a lazy bytestring, then it's split up
+and copied into strict bytestring chunks. But those get bundled
+back up into larger memory allocations as explained there. Then the files
+that are not git-annex objects are filtered out, resulting in memory
+fragmentation.
+
+I tried throwing in some S.copy in the borg list and filter part. Didn't
+help.
+
+I converted ImportLocation to use a ShortByteString, and that solved, 
+or at least improved, the borg list part of the problem. With 20 borg
+archives with 10000 annex objects each, the heap profile which had
+showed around 90 mb, mostly PINNED during that first stage, went down
+to 8 mb, none PINNED. (Although looking at the git-annex process 
+from outside, it still allocated 120 mb or so.)
+
+That leaves the memory use when constructing the git tree.
+Which would also probably affect importtree special remotes,
+when they have a large number of files.
+"""]]