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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=pbonzini@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 11/19/25 18:28, konrad.schwarz@gmail.com wrote: > The mkemmc.sh script calculates file sizes via `wc -c'. `wc' > normally works by reading the entire file, resulting in O(n) performance. Even something as mundane as 'wc' can surprise you! Running "strace wc -c < somefile.txt" shows this: fstat(0, {st_mode=S_IFREG|0644, st_size=6900, ...}) = 0 lseek(0, 0, SEEK_CUR) = 0 lseek(0, 6900, SEEK_CUR) = 6900 So wc notices you don't need word or line counts, and takes a shortcut. Paolo > Unix file systems obviously know a file's size and POSIX `ls' reports this > information unambiguously, so replacing `wc' with `ls' ensures O(1) > performance. The files in question tend to be large making this change > worthwhile. > > Signed-off-by: Konrad Schwarz > --- > scripts/mkemmc.sh | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > diff --git a/scripts/mkemmc.sh b/scripts/mkemmc.sh > index 45dc3f08fa..d2c4e84b16 100755 > --- a/scripts/mkemmc.sh > +++ b/scripts/mkemmc.sh > @@ -37,13 +37,19 @@ usage() { > exit "$1" > } > > +file_size() { > + ls_line=$(ls -Hdog "$1") || return > + printf %s\\n "$ls_line" | cut -d\ -f3 > + unset ls_line > +} > + > process_size() { > name=$1 > image_file=$2 > alignment=$3 > image_arg=$4 > if [ "${image_arg#*:}" = "$image_arg" ]; then > - if ! size=$(wc -c < "$image_file" 2>/dev/null); then > + if ! size=$(file_size "$image_file"); then > echo "Missing $name image '$image_file'." >&2 > exit 1 > fi > @@ -105,7 +111,7 @@ check_truncation() { > if [ "$image_file" = "/dev/zero" ]; then > return > fi > - if ! actual_size=$(wc -c < "$image_file" 2>/dev/null); then > + if ! actual_size=$(file_size "$image_file"); then > echo "Missing image '$image_file'." >&2 > exit 1 > fi