Subject: Re: [PATCH 1/1] upload-pack.c: make output buffer size configurable

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Tue, 14 Dec 2021 13:08:55 +0100

On Mon, Dec 13 2021, Jacob Vosmaer wrote:

> Add a new compile time constant UPLOAD_PACK_BUFFER_SIZE that makes the
> size of the static buffer in output_state configurable.
>
> The current size of the buffer is 8192+1. The '+1' is a technical
> detail orthogonal to this change. On GitLab.com we use GitLab's
> pack-objects cache which does writes of 65515 bytes. Because of the
> default 8KB buffer size, propagating these cache writes requires 8
> pipe reads and 8 pipe writes from git-upload-pack, and 8 pipe reads
> from Gitaly (our Git RPC service). If we increase the size of the
> buffer to the maximum Git packet size, we need only 1 pipe read and 1
> pipe write in git-upload-pack, and 1 pipe read in Gitaly to transfer
> the same amount of data. In benchmarks with a pure fetch and 100%
> cache hit rate workload we are seeing CPU utilization reductions of
> over 30%.
>
> Signed-off-by: Jacob Vosmaer <[email-masked]>
> ---
>  upload-pack.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/upload-pack.c b/upload-pack.c
> index c78d55bc67..b799fbe628 100644
> --- a/upload-pack.c
> +++ b/upload-pack.c
> @@ -42,6 +42,10 @@
>  #define ALL_FLAGS (THEY_HAVE | OUR_REF | WANTED | COMMON_KNOWN | SHALLOW | \
>  		NOT_SHALLOW | CLIENT_SHALLOW | HIDDEN_REF)
>  
> +#ifndef UPLOAD_PACK_BUFFER_SIZE
> +#define UPLOAD_PACK_BUFFER_SIZE 8192
> +#endif
> +
>  /* Enum for allowed unadvertised object request (UOR) */
>  enum allow_uor {
>  	/* Allow specifying sha1 if it is a ref tip. */
> @@ -194,7 +198,7 @@ static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
>  }
>  
>  struct output_state {
> -	char buffer[8193];
> +	char buffer[UPLOAD_PACK_BUFFER_SIZE+1];
>  	int used;
>  	unsigned packfile_uris_started : 1;
>  	unsigned packfile_started : 1;

Making this configurable obviousl has big impact in some cases, but I'm
a bit iffy on the faciltity to do so + it not being documented.

I don't think that the "static buffer" part here is important to anyone,
but the write() size is clearly important.

So doesn't it make more sense to have a uploadPack.bufferSize=8k
variable we can tweak, just make this "buffer" a "struct strbuf" instead
(i.e. it'll by dynamically grown), and then just flush it whenever we
hit the configured buffer size?