changeset 30451:41a8106789ca

util: implement zstd compression engine Now that zstd is vendored and being built (in some configurations), we can implement a compression engine for zstd! The zstd engine is a little different from existing engines. Because it may not always be present, we have to defer load the module in case importing it fails. We facilitate this via a cached property that holds a reference to the module or None. The "available" method is implemented to reflect reality. The zstd engine declares its ability to handle bundles using the "zstd" human name and the "ZS" internal name. The latter was chosen because internal names are 2 characters (by only convention I think) and "ZS" seems reasonable. The engine, like others, supports specifying the compression level. However, there are no consumers of this API that yet pass in that argument. I have plans to change that, so stay tuned. Since all we need to do to support bundle generation with a new compression engine is implement and register the compression engine, bundle generation with zstd "just works!" Tests demonstrating this have been added. How does performance of zstd for bundle generation compare? On the mozilla-unified repo, `hg bundle --all -t <engine>-v2` yields the following on my i7-6700K on Linux: engine CPU time bundle size vs orig size throughput none 97.0s 4,054,405,584 100.0% 41.8 MB/s bzip2 (l=9) 393.6s 975,343,098 24.0% 10.3 MB/s gzip (l=6) 184.0s 1,140,533,074 28.1% 22.0 MB/s zstd (l=1) 108.2s 1,119,434,718 27.6% 37.5 MB/s zstd (l=2) 111.3s 1,078,328,002 26.6% 36.4 MB/s zstd (l=3) 113.7s 1,011,823,727 25.0% 35.7 MB/s zstd (l=4) 116.0s 1,008,965,888 24.9% 35.0 MB/s zstd (l=5) 121.0s 977,203,148 24.1% 33.5 MB/s zstd (l=6) 131.7s 927,360,198 22.9% 30.8 MB/s zstd (l=7) 139.0s 912,808,505 22.5% 29.2 MB/s zstd (l=12) 198.1s 854,527,714 21.1% 20.5 MB/s zstd (l=18) 681.6s 789,750,690 19.5% 5.9 MB/s On compression, zstd for bundle generation delivers: * better compression than gzip with significantly less CPU utilization * better than bzip2 compression ratios while still being significantly faster than gzip * ability to aggressively tune compression level to achieve significantly smaller bundles That last point is important. With clone bundles, a server can pre-generate a bundle file, upload it to a static file server, and redirect clients to transparently download it during clone. The server could choose to produce a zstd bundle with the highest compression settings possible. This would take a very long time - a magnitude longer than a typical zstd bundle generation - but the result would be hundreds of megabytes smaller! For the clone volume we do at Mozilla, this could translate to petabytes of bandwidth savings per year and faster clones (due to smaller transfer size). I don't have detailed numbers to report on decompression. However, zstd decompression is fast: >1 GB/s output throughput on this machine, even through the Python bindings. And it can do that regardless of the compression level of the input. By the time you have enough data to worry about overhead of decompression, you have plenty of other things to worry about performance wise. zstd is wins all around. I can't wait to implement support for it on the wire protocol and in revlogs.
author Gregory Szorc <gregory.szorc@gmail.com>
date Fri, 11 Nov 2016 01:10:07 -0800
parents de48d3a0573a
children 4e1eab73c53d
files mercurial/util.py tests/test-bundle-type.t
diffstat 2 files changed, 89 insertions(+), 8 deletions(-) [+]
line wrap: on
line diff
--- a/mercurial/util.py	Thu Nov 10 23:38:41 2016 -0800
+++ b/mercurial/util.py	Fri Nov 11 01:10:07 2016 -0800
@@ -3197,5 +3197,50 @@
 
 compengines.register(_noopengine())
 
+class _zstdengine(compressionengine):
+    def name(self):
+        return 'zstd'
+
+    @propertycache
+    def _module(self):
+        # Not all installs have the zstd module available. So defer importing
+        # until first access.
+        try:
+            from . import zstd
+            # Force delayed import.
+            zstd.__version__
+            return zstd
+        except ImportError:
+            return None
+
+    def available(self):
+        return bool(self._module)
+
+    def bundletype(self):
+        return 'zstd', 'ZS'
+
+    def compressstream(self, it, opts=None):
+        opts = opts or {}
+        # zstd level 3 is almost always significantly faster than zlib
+        # while providing no worse compression. It strikes a good balance
+        # between speed and compression.
+        level = opts.get('level', 3)
+
+        zstd = self._module
+        z = zstd.ZstdCompressor(level=level).compressobj()
+        for chunk in it:
+            data = z.compress(chunk)
+            if data:
+                yield data
+
+        yield z.flush()
+
+    def decompressorreader(self, fh):
+        zstd = self._module
+        dctx = zstd.ZstdDecompressor()
+        return chunkbuffer(dctx.read_from(fh))
+
+compengines.register(_zstdengine())
+
 # convenient shortcut
 dst = debugstacktrace
--- a/tests/test-bundle-type.t	Thu Nov 10 23:38:41 2016 -0800
+++ b/tests/test-bundle-type.t	Fri Nov 11 01:10:07 2016 -0800
@@ -35,17 +35,21 @@
 
 test bundle types
 
-  $ for t in "None" "bzip2" "gzip" "none-v2" "v2" "v1" "gzip-v1"; do
-  >   echo % test bundle type $t
-  >   hg init t$t
+  $ testbundle() {
+  >   echo % test bundle type $1
+  >   hg init t$1
   >   cd t1
-  >   hg bundle -t $t ../b$t ../t$t
-  >   f -q -B6 -D ../b$t; echo
-  >   cd ../t$t
-  >   hg debugbundle ../b$t
-  >   hg debugbundle --spec ../b$t
+  >   hg bundle -t $1 ../b$1 ../t$1
+  >   f -q -B6 -D ../b$1; echo
+  >   cd ../t$1
+  >   hg debugbundle ../b$1
+  >   hg debugbundle --spec ../b$1
   >   echo
   >   cd ..
+  > }
+
+  $ for t in "None" "bzip2" "gzip" "none-v2" "v2" "v1" "gzip-v1"; do
+  >   testbundle $t
   > done
   % test bundle type None
   searching for changes
@@ -106,6 +110,38 @@
   c35a0f9217e65d1fdb90c936ffa7dbe679f83ddf
   gzip-v1
   
+#if zstd
+
+  $ for t in "zstd" "zstd-v2"; do
+  >   testbundle $t
+  > done
+  % test bundle type zstd
+  searching for changes
+  1 changesets found
+  HG20\x00\x00 (esc)
+  Stream params: sortdict([('Compression', 'ZS')])
+  changegroup -- "sortdict([('version', '02'), ('nbchanges', '1')])"
+      c35a0f9217e65d1fdb90c936ffa7dbe679f83ddf
+  zstd-v2
+  
+  % test bundle type zstd-v2
+  searching for changes
+  1 changesets found
+  HG20\x00\x00 (esc)
+  Stream params: sortdict([('Compression', 'ZS')])
+  changegroup -- "sortdict([('version', '02'), ('nbchanges', '1')])"
+      c35a0f9217e65d1fdb90c936ffa7dbe679f83ddf
+  zstd-v2
+  
+#else
+
+zstd is a valid engine but isn't available
+
+  $ hg -R t1 bundle -a -t zstd irrelevant.hg
+  abort: compression engine zstd could not be loaded
+  [255]
+
+#endif
 
 test garbage file