Subject: Re: [PATCH 6/7] reftable/record: use scratch buffer when decoding records

From: Josh Steadmon <[email-masked]>
Date: Mon, 11 Mar 2024 12:40:06 -0700

On 2024.03.05 13:11, Patrick Steinhardt wrote:
> When decoding log records we need a temporary buffer to decode the
> reflog entry's name, mail address and message. As this buffer is local
> to the function we thus have to reallocate it for every single log
> record which we're about to decode, which is inefficient.
> 
> Refactor the code such that callers need to pass in a scratch buffer,
> which allows us to reuse it for multiple decodes. This reduces the
> number of allocations when iterating through reflogs. Before:
> 
>     HEAP SUMMARY:
>         in use at exit: 13,473 bytes in 122 blocks
>       total heap usage: 2,068,487 allocs, 2,068,365 frees, 305,122,946 bytes allocated
> 
> After:
> 
>     HEAP SUMMARY:
>         in use at exit: 13,473 bytes in 122 blocks
>       total heap usage: 1,068,485 allocs, 1,068,363 frees, 281,122,886 bytes allocated
> 
> Note that this commit also drop some redundant calls to `strbuf_reset()`
> right before calling `decode_string()`. The latter already knows to
> reset the buffer, so there is no need for these.
> 
> Signed-off-by: Patrick Steinhardt <[email-masked]>
> ---
>  reftable/block.c       |  4 ++-
>  reftable/block.h       |  2 ++
>  reftable/record.c      | 52 ++++++++++++++++++--------------------
>  reftable/record.h      |  5 ++--
>  reftable/record_test.c | 57 ++++++++++++++++++++++++++----------------
>  5 files changed, 68 insertions(+), 52 deletions(-)

At first glance I was feeling somewhat negatively about this change, as
keeping persistent scratch space buffers means that either the owners
or the users of the scratch space need to be more careful about making
sure it's reset and that they're not accumulating cruft in between
various calls.

However, it appears we already have similar scratch space usage in the
sideband and cat-file code, so I guess we are OK with the pattern in
general, and we can rely on tests to make sure things are good in
practice.