[pacman-dev] [PATCH] HACKING: add some notes /proc/sys/vm/drop_caches

Xavier shiningxc at gmail.com
Tue Nov 17 12:41:58 EST 2009

On Tue, Nov 17, 2009 at 12:44 AM, Laszlo Papp <djszapi2 at gmail.com> wrote:
> Signed-off-by: Laszlo Papp <djszapi at archlinux.us>
> ---
>  HACKING |    8 ++++++++
>  1 files changed, 8 insertions(+), 0 deletions(-)
> diff --git a/HACKING b/HACKING
> index 537d9a3..63ccf1e 100644
> --- a/HACKING
> +++ b/HACKING
> @@ -146,6 +146,14 @@ For example, to run valgrind:
>     ./src/pacman/pacman
>     valgrind --leak-check=full -- src/pacman/.libs/lt-pacman -Syu
> +How should I make benchmarks?
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +pacman spends most time with reading and writing the database. because of this,
> +it's important to drop caches before doing a benchmark. here is how to do so:
> +
> +# echo 3 > /proc/sys/vm/drop_caches
> +

This is a completely retarded statement.

1) pacman spends most time with reading and writing the database


This depends whether it is cached or not. In a real world situation,
you will have only one uncached run of pacman (the first one) compared
to many cached.
It also depends on the operation. If you are going to
download/install/remove a lot of (or big) packages, it's likely
completely wrong.

2) because of this, t's important to drop caches before doing a benchmark


This depends on WTF you are benchmarking in the first place.
If you want to see the effect of your patch on the database
reading/writing, you probably want to drop caches.
If you want to see the effect on the rest, you probably do not want to
drop caches.

This tip to drop caches can sometimes be very useful, so it might be
worth mentioning in the HACKING file. And pacman database is indeed
quite inefficient (the first run on my laptop is painfully slow).

But can you please *think* about what you are writing twice (or more)
before sending it ? And common sense helps too, did you ever think
about getting some ?

I enjoy this list a lot and communicating with everyone else here.
I don't know how you manage to piss me off with every mail you send.

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