Re: [PATCH v2] fetch-pack: grow stateless RPC windows exponentially

From: Stefan Beller <sbeller@google.com>
Date: Jul 19, 2016, 16:46 UTC
Message-ID: <CAGZ79kY+2PYx9oz9tvi0zG-oE6qS-Za7D3ocY1XtqcSsDchz0Q@mail.gmail.com>
In-Reply-To: <1468880498-30235-1-git-send-email-jonathantanmy@google.com>

On Mon, Jul 18, 2016 at 3:21 PM, Jonathan Tan <jonathantanmy@google.com> wrote:

Show 5 quoted lines

> When updating large repositories, the LARGE_FLUSH limit (that is, the
> limit at which the window growth strategy switches from exponential to
> linear) is reached quite quickly. Use a conservative exponential growth
> strategy when that limit is reached instead (and increase LARGE_FLUSH so
> that there is no regression in window size).

Care to elaborate on why you choose 11/10 as growth factor?

(As someone who has a tick in micro optimizing: 9/8 is roughly the same exponent, but the division by 8 is easier as it is just a shift by 3. Similar 17/16)

I guess one design criterion was 10 being a round number? Does it make sense to experiment with the factor at all? Digging into that, LARGE_FLUSH originates from 6afca450c3f, (2011-03-20, fetch-pack: progressively use larger handshake windows), and before we only had a linear growth.

So I guess what I do not understand is why we need to slow down the exponential growth at all?

Thanks, Stefan

Show 46 quoted lines

>
> This optimization is only applied during stateless RPCs to avoid the
> issue raised and fixed in commit
> 44d8dc54e73e8010c4bdf57a422fc8d5ce709029.
>
> Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
> ---
>  fetch-pack.c | 19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
>
> diff --git a/fetch-pack.c b/fetch-pack.c
> index b501d5c..85e77af 100644
> --- a/fetch-pack.c
> +++ b/fetch-pack.c
> @@ -243,16 +243,21 @@ static void insert_one_alternate_ref(const struct ref *ref, void *unused)
>
>  #define INITIAL_FLUSH 16
>  #define PIPESAFE_FLUSH 32
> -#define LARGE_FLUSH 1024
> +#define LARGE_FLUSH 16384
>
>  static int next_flush(struct fetch_pack_args *args, int count)
>  {
> -       int flush_limit = args->stateless_rpc ? LARGE_FLUSH : PIPESAFE_FLUSH;
> -
> -       if (count < flush_limit)
> -               count <<= 1;
> -       else
> -               count += flush_limit;
> +       if (args->stateless_rpc) {
> +               if (count < LARGE_FLUSH)
> +                       count <<= 1;
> +               else
> +                       count = count * 11 / 10;
> +       } else {
> +               if (count < PIPESAFE_FLUSH)
> +                       count <<= 1;
> +               else
> +                       count += PIPESAFE_FLUSH;
> +       }
>         return count;
>  }
>
> --
> 2.8.0.rc3.226.g39d4020
>