Re: [Libguestfs] [nbdkit PATCH] sh: Prefer dd bs=1 over iflag=count_bytes
On 8/17/20 11:35 AM, Eric Blake wrote:
While iflag=count_bytes combined with bs > 1 allows for more
efficient
operation, it is a feature of GNU dd, and not present on other
implementations such as BSD. Sticking to just POSIX features makes
things more portable.
Signed-off-by: Eric Blake <eblake(a)redhat.com>
---
docs/nbdkit-loop.pod | 2 +-
docs/nbdkit.pod | 4 ++--
plugins/eval/nbdkit-eval-plugin.pod | 6 +++---
plugins/sh/nbdkit-sh-plugin.pod | 2 +-
plugins/sh/assemble.sh | 2 +-
plugins/sh/example.sh | 7 +++----
tests/test-cacheextents.sh | 5 ++---
tests/test-eval-file.sh | 5 ++---
tests/test-eval.sh | 3 +--
tests/test-export-name.sh | 5 ++---
tests/test-parallel-sh.sh | 5 ++---
tests/test-readahead-test-plugin.sh | 2 +-
tests/test-readahead.sh | 1 -
tests/test-retry-reopen-fail.sh | 3 +--
tests/test-retry-size.sh | 3 +--
tests/test-retry.sh | 3 +--
tests/test-shell.sh | 5 ++---
tests/test-single-sh.sh | 3 +--
tests/test-tls-fallback.sh | 2 +-
19 files changed, 28 insertions(+), 40 deletions(-)
To see if this makes a noticeable impact on GNU systems, I compared:
$ time make -C tests check TESTS='test-cacheextents.sh test-eval-file.sh
test-eval.sh test-export-name.sh test-parallel-sh.sh test-readahead.sh
test-retry-reopen-fail.sh test-retry-size.sh test-retry.sh test-shell
test-single-sh.sh test-tls-fallback.sh'
after first completing an entire 'make check' to prime the cache.
Typical timings of a few runs were
before:
real 1m45.445s
user 0m6.626s
sys 0m4.181s
after:
real 1m53.366s
user 0m13.510s
sys 0m13.387s
So it is indeed a noticeable slowdown (although it also points out that
the bulk of our test time is spend sleep()ing, waiting for servers to
start or for parallel tests to prove their merit).
But not all the tests use the same amount of .pread calls; breaking it
down into per-test timings, I see the worst degradation in
test-eval-file.sh (8 seconds!), second worst in test-shell (~0.8s); but
both make sense - those are the tests where we are hammering the file
with the most .pread and .pwrite calls from libguestfs, rather than a
one-off read or even testing something unrelated to read.
Maybe instead of directly using this patch, we instead compromise by
mentioning the speedup trick in the .pod files (list the portable
spelling side-by-side with the efficient GNU-only spelling), and make
those two tests use the efficient spelling.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org