Re: [PATCHv2] refs.c: enable large transactions

[Michael Haggerty <mhagger@alum.mit.edu>]

On 04/21/2015 09:06 PM, Stefan Beller wrote:
> This is another attempt on enabling large transactions
> (large in terms of open file descriptors). We keep track of how many
> lock files are opened by the ref_transaction_commit function.
> When more than a reasonable amount of files is open, we close
> the file descriptors to make sure the transaction can continue.
> 
> Another idea I had during implementing this was to move this file
> closing into the lock file API, such that only a certain amount of
> lock files can be open at any given point in time and we'd be 'garbage
> collecting' open fds when necessary in any relevant call to the lock
> file API. This would have brought the advantage of having such
> functionality available in other users of the lock file API as well.
> The downside however is the over complication, you really need to always
> check for (lock->fd != -1) all the time, which may slow down other parts
> of the code, which did not ask for such a feature.

Aside from a missing error check (see below), this looks good to me.

> Signed-off-by: Stefan Beller <sbeller@google.com>
> ---
> 
> * Removed unneeded braces in the condition to check if we want to close
>   the lock file.
> * made the counter for the remaining fds an unsigned int. That is what   
>   get_max_fd_limit() returns, so there are no concerns for an overflow.
>   Also it cannot go below 0 any more.
> * moved the initialisation of the remaining_fds a bit down and added a comment  
>   
>  refs.c                | 21 +++++++++++++++++++++
>  t/t1400-update-ref.sh |  4 ++--
>  2 files changed, 23 insertions(+), 2 deletions(-)
>  
>  
> 
> diff --git a/refs.c b/refs.c
> index 4f495bd..34cfcdf 100644
> --- a/refs.c
> +++ b/refs.c
> @@ -3041,6 +3041,8 @@ static int write_ref_sha1(struct ref_lock *lock,
>  		errno = EINVAL;
>  		return -1;
>  	}
> +	if (lock->lk->fd == -1)
> +		reopen_lock_file(lock->lk);

You should check that reopen_lock_file() was successful.

>  	if (write_in_full(lock->lk->fd, sha1_to_hex(sha1), 40) != 40 ||
>  	    write_in_full(lock->lk->fd, &term, 1) != 1 ||
>  	    close_ref(lock) < 0) {
> @@ -3718,6 +3720,7 @@ int ref_transaction_commit(struct ref_transaction *transaction,
>  			   struct strbuf *err)
>  {
>  	int ret = 0, i;
> +	unsigned int remaining_fds;
>  	int n = transaction->nr;
>  	struct ref_update **updates = transaction->updates;
>  	struct string_list refs_to_delete = STRING_LIST_INIT_NODUP;
> @@ -3733,6 +3736,20 @@ int ref_transaction_commit(struct ref_transaction *transaction,
>  		return 0;
>  	}
>  
> +	/*
> +	 * We need to open many files in a large transaction, so come up with
> +	 * a reasonable maximum. We still keep some spares for stdin/out and
> +	 * other open files. Experiments determined we need more fds when
> +	 * running inside our test suite than directly in the shell. It's
> +	 * unclear where these fds come from. 32 should be a reasonable large
> +	 * number though.
> +	 */
> +	remaining_fds = get_max_fd_limit();
> +	if (remaining_fds > 32)
> +		remaining_fds -= 32;
> +	else
> +		remaining_fds = 0;
> +
>  	/* Copy, sort, and reject duplicate refs */
>  	qsort(updates, n, sizeof(*updates), ref_update_compare);
>  	if (ref_update_reject_duplicates(updates, n, err)) {
> @@ -3762,6 +3779,10 @@ int ref_transaction_commit(struct ref_transaction *transaction,
>  				    update->refname);
>  			goto cleanup;
>  		}
> +		if (remaining_fds > 0)
> +			remaining_fds--;
> +		else
> +			close_lock_file(update->lock->lk);

I consider this code a stopgap, and simplicity is more important than
optimization. But just for the sake of discussion, if we planned to keep
this code around, it could be improved by not wasting open file
descriptors for references that are only being verified or deleted, like so:

                if (!(flags & REF_HAVE_NEW) ||
                    is_null_sha1(update->new_sha1) ||
                    remaining_fds == 0)
                        close_lock_file(update->lock->lk);
                else
                        remaining_fds--;

>  	}
>  
>  	/* Perform updates first so live commits remain referenced */
> [...]

Michael

-- 
Michael Haggerty
mhagger@alum.mit.edu