Patchwork [V3] util: improve iterfile so it chooses code path wisely

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Submitter Jun Wu
Date Nov. 16, 2016, 4:17 p.m.
Message ID <232935ca927d68bd5bed.1479313023@x1c>
Download mbox | patch
Permalink /patch/17599/
State Accepted
Headers show

Comments

Jun Wu - Nov. 16, 2016, 4:17 p.m.
# HG changeset patch
# User Jun Wu <quark@fb.com>
# Date 1479241551 0
#      Tue Nov 15 20:25:51 2016 +0000
# Node ID 232935ca927d68bd5bed66b674dfd58cbb13805d
# Parent  d1a0a64f6e16432333bea0476098c46a61222b9b
# Available At https://bitbucket.org/quark-zju/hg-draft
#              hg pull https://bitbucket.org/quark-zju/hg-draft -r 232935ca927d
util: improve iterfile so it chooses code path wisely

We have performance concerns on "iterfile" as it is 4X slower on normal
files. While modern systems have the nice property that reading a "fast"
(on-disk) file cannot be interrupted and should be made use of.

This patch dumps the related knowledge in comments. And "iterfile" chooses
code paths wisely:

  1. If it's CPython 3, or PyPY, use the fast path.
  2. If fp is a normal file, use the fast path.
  3. If fp is not a normal file and CPython version >= 2.7.4, use the same
     workaround (4x slower) as before.
  4. If fp is not a normal file and CPython version < 2.7.4, use another
     workaround (2x slower but may block longer then necessary) which
     basically re-invents the buffer + readline logic in Python.

This will give us good confidence on both correctness and performance
dealing with EINTR in iterfile(fp) for all known supported Python versions.
Augie Fackler - Nov. 16, 2016, 10:56 p.m.
On Wed, Nov 16, 2016 at 04:17:03PM +0000, Jun Wu wrote:
> # HG changeset patch
> # User Jun Wu <quark@fb.com>
> # Date 1479241551 0
> #      Tue Nov 15 20:25:51 2016 +0000
> # Node ID 232935ca927d68bd5bed66b674dfd58cbb13805d
> # Parent  d1a0a64f6e16432333bea0476098c46a61222b9b
> # Available At https://bitbucket.org/quark-zju/hg-draft
> #              hg pull https://bitbucket.org/quark-zju/hg-draft -r 232935ca927d
> util: improve iterfile so it chooses code path wisely

Queued. Sigh.

>
> We have performance concerns on "iterfile" as it is 4X slower on normal
> files. While modern systems have the nice property that reading a "fast"
> (on-disk) file cannot be interrupted and should be made use of.
>
> This patch dumps the related knowledge in comments. And "iterfile" chooses
> code paths wisely:
>
>   1. If it's CPython 3, or PyPY, use the fast path.
>   2. If fp is a normal file, use the fast path.
>   3. If fp is not a normal file and CPython version >= 2.7.4, use the same
>      workaround (4x slower) as before.
>   4. If fp is not a normal file and CPython version < 2.7.4, use another
>      workaround (2x slower but may block longer then necessary) which
>      basically re-invents the buffer + readline logic in Python.
>
> This will give us good confidence on both correctness and performance
> dealing with EINTR in iterfile(fp) for all known supported Python versions.
>
> diff --git a/mercurial/util.py b/mercurial/util.py
> --- a/mercurial/util.py
> +++ b/mercurial/util.py
> @@ -25,8 +25,10 @@ import hashlib
>  import imp
>  import os
> +import platform as pyplatform
>  import re as remod
>  import shutil
>  import signal
>  import socket
> +import stat
>  import string
>  import subprocess
> @@ -2191,8 +2193,75 @@ def wrap(line, width, initindent='', han
>      return wrapper.fill(line).encode(encoding.encoding)
>
> -def iterfile(fp):
> -    """like fp.__iter__ but does not have issues with EINTR. Python 2.7.12 is
> -    known to have such issues."""
> -    return iter(fp.readline, '')
> +if (pyplatform.python_implementation() == 'CPython' and
> +    sys.version_info < (3, 0)):
> +    # There is an issue in CPython that some IO methods do not handle EINTR
> +    # correctly. The following table shows what CPython version (and functions)
> +    # are affected (buggy: has the EINTR bug, okay: otherwise):
> +    #
> +    #                | < 2.7.4 | 2.7.4 to 2.7.12 | >= 3.0
> +    #   --------------------------------------------------
> +    #    fp.__iter__ | buggy   | buggy           | okay
> +    #    fp.read*    | buggy   | okay [1]        | okay
> +    #
> +    # [1]: fixed by changeset 67dc99a989cd in the cpython hg repo.
> +    #
> +    # Here we workaround the EINTR issue for fileobj.__iter__. Other methods
> +    # like "read*" are ignored for now, as Python < 2.7.4 is a minority.
> +    #
> +    # Although we can workaround the EINTR issue for fp.__iter__, it is slower:
> +    # "for x in fp" is 4x faster than "for x in iter(fp.readline, '')" in
> +    # CPython 2, because CPython 2 maintains an internal readahead buffer for
> +    # fp.__iter__ but not other fp.read* methods.
> +    #
> +    # On modern systems like Linux, the "read" syscall cannot be interrupted
> +    # when reading "fast" files like on-disk files. So the EINTR issue only
> +    # affects things like pipes, sockets, ttys etc. We treat "normal" (S_ISREG)
> +    # files approximately as "fast" files and use the fast (unsafe) code path,
> +    # to minimize the performance impact.
> +    if sys.version_info >= (2, 7, 4):
> +        # fp.readline deals with EINTR correctly, use it as a workaround.
> +        def _safeiterfile(fp):
> +            return iter(fp.readline, '')
> +    else:
> +        # fp.read* are broken too, manually deal with EINTR in a stupid way.
> +        # note: this may block longer than necessary because of bufsize.
> +        def _safeiterfile(fp, bufsize=4096):
> +            fd = fp.fileno()
> +            line = ''
> +            while True:
> +                try:
> +                    buf = os.read(fd, bufsize)
> +                except OSError as ex:
> +                    # os.read only raises EINTR before any data is read
> +                    if ex.errno == errno.EINTR:
> +                        continue
> +                    else:
> +                        raise
> +                line += buf
> +                if '\n' in buf:
> +                    splitted = line.splitlines(True)
> +                    line = ''
> +                    for l in splitted:
> +                        if l[-1] == '\n':
> +                            yield l
> +                        else:
> +                            line = l
> +                if not buf:
> +                    break
> +            if line:
> +                yield line
> +
> +    def iterfile(fp):
> +        fastpath = True
> +        if type(fp) is file:
> +            fastpath = stat.S_ISREG(os.fstat(fp.fileno()).st_mode)
> +        if fastpath:
> +            return fp
> +        else:
> +            return _safeiterfile(fp)
> +else:
> +    # PyPy and CPython 3 do not have the EINTR issue thus no workaround needed.
> +    def iterfile(fp):
> +        return fp
>
>  def iterlines(iterator):
> _______________________________________________
> Mercurial-devel mailing list
> Mercurial-devel@mercurial-scm.org
> https://www.mercurial-scm.org/mailman/listinfo/mercurial-devel

Patch

diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -25,8 +25,10 @@  import hashlib
 import imp
 import os
+import platform as pyplatform
 import re as remod
 import shutil
 import signal
 import socket
+import stat
 import string
 import subprocess
@@ -2191,8 +2193,75 @@  def wrap(line, width, initindent='', han
     return wrapper.fill(line).encode(encoding.encoding)
 
-def iterfile(fp):
-    """like fp.__iter__ but does not have issues with EINTR. Python 2.7.12 is
-    known to have such issues."""
-    return iter(fp.readline, '')
+if (pyplatform.python_implementation() == 'CPython' and
+    sys.version_info < (3, 0)):
+    # There is an issue in CPython that some IO methods do not handle EINTR
+    # correctly. The following table shows what CPython version (and functions)
+    # are affected (buggy: has the EINTR bug, okay: otherwise):
+    #
+    #                | < 2.7.4 | 2.7.4 to 2.7.12 | >= 3.0
+    #   --------------------------------------------------
+    #    fp.__iter__ | buggy   | buggy           | okay
+    #    fp.read*    | buggy   | okay [1]        | okay
+    #
+    # [1]: fixed by changeset 67dc99a989cd in the cpython hg repo.
+    #
+    # Here we workaround the EINTR issue for fileobj.__iter__. Other methods
+    # like "read*" are ignored for now, as Python < 2.7.4 is a minority.
+    #
+    # Although we can workaround the EINTR issue for fp.__iter__, it is slower:
+    # "for x in fp" is 4x faster than "for x in iter(fp.readline, '')" in
+    # CPython 2, because CPython 2 maintains an internal readahead buffer for
+    # fp.__iter__ but not other fp.read* methods.
+    #
+    # On modern systems like Linux, the "read" syscall cannot be interrupted
+    # when reading "fast" files like on-disk files. So the EINTR issue only
+    # affects things like pipes, sockets, ttys etc. We treat "normal" (S_ISREG)
+    # files approximately as "fast" files and use the fast (unsafe) code path,
+    # to minimize the performance impact.
+    if sys.version_info >= (2, 7, 4):
+        # fp.readline deals with EINTR correctly, use it as a workaround.
+        def _safeiterfile(fp):
+            return iter(fp.readline, '')
+    else:
+        # fp.read* are broken too, manually deal with EINTR in a stupid way.
+        # note: this may block longer than necessary because of bufsize.
+        def _safeiterfile(fp, bufsize=4096):
+            fd = fp.fileno()
+            line = ''
+            while True:
+                try:
+                    buf = os.read(fd, bufsize)
+                except OSError as ex:
+                    # os.read only raises EINTR before any data is read
+                    if ex.errno == errno.EINTR:
+                        continue
+                    else:
+                        raise
+                line += buf
+                if '\n' in buf:
+                    splitted = line.splitlines(True)
+                    line = ''
+                    for l in splitted:
+                        if l[-1] == '\n':
+                            yield l
+                        else:
+                            line = l
+                if not buf:
+                    break
+            if line:
+                yield line
+
+    def iterfile(fp):
+        fastpath = True
+        if type(fp) is file:
+            fastpath = stat.S_ISREG(os.fstat(fp.fileno()).st_mode)
+        if fastpath:
+            return fp
+        else:
+            return _safeiterfile(fp)
+else:
+    # PyPy and CPython 3 do not have the EINTR issue thus no workaround needed.
+    def iterfile(fp):
+        return fp
 
 def iterlines(iterator):