Re: cp(1) fails to copy file from /proc

[Jim Meyering]

Jim Meyering wrote:
...
> I've confirmed this on the following kernels:
>
>   debian unstable  2.6.26-2-amd64
>   fedora 10        2.6.27.21-170.2.56.fc10.x86_64
>   ubuntu           2.6.28-11-generic
>   fedora rawhide   2.6.29.1-70.fc11.x86_64

The misbehavior is reproducible on 2.6.9-78.0.17.ELsmp, too.

But on 2.4.9-e.62smp, (yep, 2.*4*) it is not a problem.

Plus I did some quick timings, using cp -r to copy a tree of 201
directories and 400,000 1-byte files (top dir, then 200 directories,
each containing 200 files).  Dir and file names were all very short,
e.g.,

  $ find z|head
  z
  z/63
  z/63/63
  z/63/54
  z/63/166
  z/63/12
  z/63/94
  z/63/186
  z/63/147
  z/63/61

I ran the tests on both an ext3 file system
and on a tmpfs file system on an idle x86_64 Fedora 10 system.

The timings suggest that the save-one-read-syscall optimization
results in no more than a 2% speed-up, in this pathological case.
In normal usage, I suspect that any speed-up would be immeasurably small.

So I have no qualms about removing the optimization.

If anyone can measure a significant regression, say on NFS,
please provide details.  Here's the patch I'm considering.
I'll see about adding a test, too.

>From 4b2a62e3b46f50ed6035fdfb7c678ec38c70e35d Mon Sep 17 00:00:00 2001
From: Jim Meyering <address@hidden>
Date: Fri, 17 Apr 2009 18:44:18 +0200
Subject: [PATCH] copy.c: remove optimization that avoided up to 50% of read 
syscalls

Do not assume that a short read on a regular file indicates EOF.
When reading from a file in /proc on linux [at least 2.6.9 - 2.6.29]
into a 4k-byte buffer or larger, a short read does not always indicate
EOF.  For example, "cp /proc/slabinfo /tmp" copies only 4068 of the
total 7493 bytes.  This optimization appears to have been worth less
than a 2% speed-up, so the impact of removing it is negligible.
* src/copy.c (copy_reg): Don't exit the loop early.
---
 src/copy.c |    4 ----
 1 files changed, 0 insertions(+), 4 deletions(-)

diff --git a/src/copy.c b/src/copy.c
index 9b0e139..3cbeba4 100644
--- a/src/copy.c
+++ b/src/copy.c
@@ -699,10 +699,6 @@ copy_reg (char const *src_name, char const *dst_name,
                goto close_src_and_dst_desc;
              }
            last_write_made_hole = false;
-
-           /* A short read on a regular file means EOF.  */
-           if (n_read != buf_size && S_ISREG (src_open_sb.st_mode))
-             break;
          }
       }

--
1.6.3.rc0.200.g4086