Why is "git fetch --prune" so much slower than "git remote prune"?

Why is "git fetch --prune" so much slower than "git remote prune"?

[Ævar Arnfjörð Bjarmason]

The --prune option to fetch added in v1.6.5-8-gf360d84 seems to be
around 20-30x slower than the equivalent operation with git remote
prune. I'm wondering if I'm missing something and fetch does something
more, but it doesn't seem so.

To test this clone git.git, create 1000 branches it in, create two
local clones of that clone and then delete the 1000 branches in the
original. I have a script to do this at
https://gist.github.com/avar/497c8c8fbd641fb756ef

Then in each of the clones:

    $ git branch -a|wc -l; time (~/g/git/git-fetch --prune origin
>/dev/null 2>&1); git branch -a | wc -l
1003
    real    0m3.337s
    user    0m2.996s
    sys     0m0.336s
    3

    $ git branch -a|wc -l; time (~/g/git/git-remote prune origin
>/dev/null 2>&1); git branch -a | wc -l
    1003
    real    0m0.067s
    user    0m0.020s
    sys     0m0.040s
    3

Both of these ends up doing a "git fetch", so it's not that. I'm quite
rusty in C profiling but here's a gprof of the git-fetch command:

$ gprof ~/g/git/git-fetch|head -n 20
Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls   s/call   s/call  name
 26.42      0.33     0.33  1584583     0.00     0.00  strbuf_getwholeline
 14.63      0.51     0.18 90601347     0.00     0.00  strbuf_grow
 13.82      0.68     0.17  1045676     0.00     0.00  find_pack_entry_one
  8.13      0.78     0.10  1050062     0.00     0.00  check_refname_format
  6.50      0.86     0.08  1584675     0.00     0.00  get_sha1_hex
  5.69      0.93     0.07  2100529     0.00     0.00  starts_with
  3.25      0.97     0.04  1044043     0.00     0.00  refname_is_safe
  3.25      1.01     0.04     8007     0.00     0.00  get_packed_ref_cache
  2.44      1.04     0.03  2605595     0.00     0.00  search_ref_dir
  2.44      1.07     0.03  1040500     0.00     0.00  peel_entry
  1.63      1.09     0.02  2632661     0.00     0.00  get_ref_dir
  1.63      1.11     0.02  1044043     0.00     0.00  create_ref_entry
  1.63      1.13     0.02     8024     0.00     0.00  do_for_each_entry_in_dir
  0.81      1.14     0.01  2155105     0.00     0.00  memory_limit_check
  0.81      1.15     0.01  1580503     0.00     0.00  sha1_to_hex

And of the git-remote command:

$ gprof ~/g/git/git-remote|head -n 20
Flat profile:

Each sample counts as 0.01 seconds.
 no time accumulated

  %   cumulative   self              self     total
 time   seconds   seconds    calls  Ts/call  Ts/call  name
  0.00      0.00     0.00   197475     0.00     0.00  strbuf_grow
  0.00      0.00     0.00    24214     0.00     0.00  sort_ref_dir
  0.00      0.00     0.00    24190     0.00     0.00  search_ref_dir
  0.00      0.00     0.00    21661     0.00     0.00  memory_limit_check
  0.00      0.00     0.00    20236     0.00     0.00  get_ref_dir
  0.00      0.00     0.00     9187     0.00     0.00  xrealloc
  0.00      0.00     0.00     7048     0.00     0.00  strbuf_add
  0.00      0.00     0.00     6348     0.00     0.00  do_xmalloc
  0.00      0.00     0.00     6126     0.00     0.00  xcalloc
  0.00      0.00     0.00     6056     0.00     0.00  cleanup_path
  0.00      0.00     0.00     6050     0.00     0.00  get_git_dir
  0.00      0.00     0.00     6050     0.00     0.00  vsnpath
  0.00      0.00     0.00     5554     0.00     0.00  config_file_fgetc

Aside from the slowness of git-fetch it seems git-remote can be sped
up quite a bit by more aggressively allocating a larger string buffer
from the get-go.

Re: Why is "git fetch --prune" so much slower than "git remote prune"?

[Jeff King]

On Fri, Mar 06, 2015 at 05:48:39PM +0100, Ævar Arnfjörð Bjarmason wrote:

> The --prune option to fetch added in v1.6.5-8-gf360d84 seems to be
> around 20-30x slower than the equivalent operation with git remote
> prune. I'm wondering if I'm missing something and fetch does something
> more, but it doesn't seem so.

"git fetch --prune" is "do a normal fetch, and also prune anything
necessary". "git remote prune" is "ls-remote the other side and see if
there is anything we can prune; do not touch anything else".

If your fetch is a noop (i.e., the other side has not advanced any
branches), the outcome is the same. But perhaps fetch is doing more
work to find out that it is a noop.

One way to measure that would be to see how expensive a noop "git fetch"
is (if it's expensive, then there is room to improve there. If not, then
it is the pruning itself that is expensive).

But just guessing (I do not have time to dig in deeper right now), and
seeing this:

> $ gprof ~/g/git/git-fetch|head -n 20
> Flat profile:
> 
> Each sample counts as 0.01 seconds.
>   %   cumulative   self              self     total
>  time   seconds   seconds    calls   s/call   s/call  name
>  26.42      0.33     0.33  1584583     0.00     0.00  strbuf_getwholeline
>  14.63      0.51     0.18 90601347     0.00     0.00  strbuf_grow
>  13.82      0.68     0.17  1045676     0.00     0.00  find_pack_entry_one
>   8.13      0.78     0.10  1050062     0.00     0.00  check_refname_format
>   6.50      0.86     0.08  1584675     0.00     0.00  get_sha1_hex
>   5.69      0.93     0.07  2100529     0.00     0.00  starts_with
>   3.25      0.97     0.04  1044043     0.00     0.00  refname_is_safe
>   3.25      1.01     0.04     8007     0.00     0.00  get_packed_ref_cache
>   2.44      1.04     0.03  2605595     0.00     0.00  search_ref_dir
>   2.44      1.07     0.03  1040500     0.00     0.00  peel_entry
>   1.63      1.09     0.02  2632661     0.00     0.00  get_ref_dir
>   1.63      1.11     0.02  1044043     0.00     0.00  create_ref_entry
>   1.63      1.13     0.02     8024     0.00     0.00  do_for_each_entry_in_dir
>   0.81      1.14     0.01  2155105     0.00     0.00  memory_limit_check
>   0.81      1.15     0.01  1580503     0.00     0.00  sha1_to_hex

We spend a lot of time checking refs here. Probably this comes from
writing the `packed-refs` file out 1000 times in your example, because
fetch handles each ref individually. Whereas since c9e768b (remote:
repack packed-refs once when deleting multiple refs, 2014-05-23),
git-remote does it in one pass.

Now that we have ref_transaction_*, I think if git-fetch fed all of the
deletes (along with the updates) into a single transaction, we would get
the same optimization for free. Maybe that is even part of some of the
pending ref_transaction work from Stefan or Michael (both cc'd). I
haven't kept up very well with what is cooking in pu.

-Peff

Re: Why is "git fetch --prune" so much slower than "git remote prune"?

[Michael Haggerty]

On 03/06/2015 11:59 PM, Jeff King wrote:
> On Fri, Mar 06, 2015 at 05:48:39PM +0100, Ævar Arnfjörð Bjarmason wrote:
> 
>> The --prune option to fetch added in v1.6.5-8-gf360d84 seems to be
>> around 20-30x slower than the equivalent operation with git remote
>> prune. I'm wondering if I'm missing something and fetch does something
>> more, but it doesn't seem so.
> 
> [...]
> We spend a lot of time checking refs here. Probably this comes from
> writing the `packed-refs` file out 1000 times in your example, because
> fetch handles each ref individually. Whereas since c9e768b (remote:
> repack packed-refs once when deleting multiple refs, 2014-05-23),
> git-remote does it in one pass.
> 
> Now that we have ref_transaction_*, I think if git-fetch fed all of the
> deletes (along with the updates) into a single transaction, we would get
> the same optimization for free. Maybe that is even part of some of the
> pending ref_transaction work from Stefan or Michael (both cc'd). I
> haven't kept up very well with what is cooking in pu.

I am looking into this now.

For pruning, we can't use a ref_transaction as it is currently
implemented because it would fail if any of the reference deletions
failed. But in this case I think if any deletions fail, we would prefer
to emit a warning but keep going.

I'm trying to decide whether to have a separate function in the refs API
to "delete as many of the following refs as possible", or whether to add
a flag to ref_transaction_update() that says "try this update, but don't
fail the transaction if it fails". The latter would probably be more
work because we would need to invent a way to return multiple error
messages from a single transaction.

Michael

-- 
Michael Haggerty
mhagger@alum.mit.edu

Re: Why is "git fetch --prune" so much slower than "git remote prune"?

[Jeff King]

On Thu, Mar 19, 2015 at 03:49:08PM +0100, Michael Haggerty wrote:

> For pruning, we can't use a ref_transaction as it is currently
> implemented because it would fail if any of the reference deletions
> failed. But in this case I think if any deletions fail, we would prefer
> to emit a warning but keep going.
>
> I'm trying to decide whether to have a separate function in the refs API
> to "delete as many of the following refs as possible", or whether to add
> a flag to ref_transaction_update() that says "try this update, but don't
> fail the transaction if it fails". The latter would probably be more
> work because we would need to invent a way to return multiple error
> messages from a single transaction.

Hmm, yeah. That is sort of antithetical to the original purpose of ref
transactions (which was atomicity). Given the way it is implemented now,
I don't think it would be too big a deal to keep a per-ref status (one
per "struct ref_upate").  But I would worry in the long run that it
makes things much harder on proposed ref backends, which otherwise can
consider a transaction an atomic unit.

OTOH, I do not think there is an abstracted way to do this _without_
the ref backend knowing about it. You cannot just do a series of
transactions; that defeats the purpose. It is a bit of a layering
violation that builtin/remote.c (which does this optimization already)
calls repack_without_refs directly.

-Peff

Re: Why is "git fetch --prune" so much slower than "git remote prune"?

[Junio C Hamano]

Michael Haggerty <mhagger@alum.mit.edu> writes:

>> Now that we have ref_transaction_*, I think if git-fetch fed all of the
>> deletes (along with the updates) into a single transaction, we would get
>> the same optimization for free. Maybe that is even part of some of the
>> pending ref_transaction work from Stefan or Michael (both cc'd). I
>> haven't kept up very well with what is cooking in pu.
>
> I am looking into this now.
>
> For pruning, we can't use a ref_transaction as it is currently
> implemented because it would fail if any of the reference deletions
> failed. But in this case I think if any deletions fail, we would prefer
> to emit a warning but keep going.

I am not quite sure what you mean here.  I agree with you if you
meant "we shouldn't fail the fetch only because 'fetch --prune'
failed to remove only one of the remote-tracking refs that are no
longer there" but that can easily be solved by the pruning phase
into a separate transaction.  If you meant "we would rather remove
origin/{a,b} non-atomically when we noticed that origin/{a,b,c} are
all gone than leaving all three intact only because we failed to
remove origin/c for whatever reason", my knee-jerk reaction is "does
it make practical difference to the end user between these two?"

What are the plausible cause of failing to prune unused
remote-tracking refs?

Re: Why is "git fetch --prune" so much slower than "git remote prune"?

[Jeff King]

On Thu, Mar 19, 2015 at 12:24:21PM -0700, Junio C Hamano wrote:

> > For pruning, we can't use a ref_transaction as it is currently
> > implemented because it would fail if any of the reference deletions
> > failed. But in this case I think if any deletions fail, we would prefer
> > to emit a warning but keep going.
> 
> I am not quite sure what you mean here.  I agree with you if you
> meant "we shouldn't fail the fetch only because 'fetch --prune'
> failed to remove only one of the remote-tracking refs that are no
> longer there" but that can easily be solved by the pruning phase
> into a separate transaction.  If you meant "we would rather remove
> origin/{a,b} non-atomically when we noticed that origin/{a,b,c} are
> all gone than leaving all three intact only because we failed to
> remove origin/c for whatever reason", my knee-jerk reaction is "does
> it make practical difference to the end user between these two?"
> 
> What are the plausible cause of failing to prune unused
> remote-tracking refs?

I had assumed earlier that Michael meant to use a single ref_transaction
for all updates. Thinking on it more, that is probably a bad idea, as it
makes fetch atomic in a user-visible way, whereas currently the updates
are always per-ref (i.e., some may fail, but we let others succeed). I
don't know if people actually care or not (certainly the exit code of
fetch represents all of the refs, so it is not like you could say "eh,
git-fetch return 1, but it probably got the ref I wanted" without
parsing the human-readable output).

If it is just a single atomic commit for all of the deletions, I agree
it is less of a big deal. They are unlikely to fail, and when they do,
you are not blocking the new refs from coming in.

I think the ref_transaction does have some smarts to handle a case where
we are updating "refs/foo/bar" while "refs/foo" exists but is deleted in
the transaction. We switched to pruning before updating in
10a6cc8 (fetch --prune: Run prune before fetching, 2014-01-02), so it is
a non-issue, but what is there now is technically racy[1], and it would
have been nice to let the ref-storage code handle it. I guess we still
can if we introduce a "git fetch --atomic" option.

-Peff

[1] http://article.gmane.org/gmane.comp.version-control.git/239519

Re: Why is "git fetch --prune" so much slower than "git remote prune"?

[Michael Haggerty]

On 03/19/2015 08:24 PM, Junio C Hamano wrote:
> Michael Haggerty <mhagger@alum.mit.edu> writes:
> 
>>> Now that we have ref_transaction_*, I think if git-fetch fed all of the
>>> deletes (along with the updates) into a single transaction, we would get
>>> the same optimization for free. Maybe that is even part of some of the
>>> pending ref_transaction work from Stefan or Michael (both cc'd). I
>>> haven't kept up very well with what is cooking in pu.
>>
>> I am looking into this now.
>>
>> For pruning, we can't use a ref_transaction as it is currently
>> implemented because it would fail if any of the reference deletions
>> failed. But in this case I think if any deletions fail, we would prefer
>> to emit a warning but keep going.
> 
> I am not quite sure what you mean here.  I agree with you if you
> meant "we shouldn't fail the fetch only because 'fetch --prune'
> failed to remove only one of the remote-tracking refs that are no
> longer there" but that can easily be solved by the pruning phase
> into a separate transaction.  If you meant "we would rather remove
> origin/{a,b} non-atomically when we noticed that origin/{a,b,c} are
> all gone than leaving all three intact only because we failed to
> remove origin/c for whatever reason", my knee-jerk reaction is "does
> it make practical difference to the end user between these two?"
> 
> What are the plausible cause of failing to prune unused
> remote-tracking refs?

I wasn't proposing to combine the updates with the prunes in a single
transaction. (Though now that I think about it, when "fetch --atomic" is
specified it wouldn't be so crazy. It would also have the virtue of
allowing the reference transaction code to deal with any D/F conflicts
between references being added and references being deleted.)

What I meant is the second thing you mentioned, namely that currently we
prune each reference on a "best-effort" basis and wouldn't skip *all*
prunes just because one failed.

But you raise an interesting question: what could cause a failure to
prune an unused remote-tracking ref (i.e. if all pruning is done in a
single transaction)?

* I think the most likely reason for a failure would be a lock conflict
with another process. We don't retry lock attempts [1], so this would
cause the whole transaction to fail. This could happen, for example, if
the user launches two "fetch --prune" operations at the same time, or
runs "pack-refs" while fetching [2].

* It is *not* a failure if another process deletes or updates the
reference before we get to it, because we don't provide an "old" value
of the reference for a pre-check.

* I haven't verified this, but I can imagine it could fail if another
process deletes the reference (say refs/remotes/origin/foo/bar) and adds
another reference that would D/F conflict with it (say
refs/remotes/origin/foo) while we are working, because our attempt to
create refs/remotes/origin/foo/bar.lock would fail.

* A repository problem, like for example wrong file permissions
somewhere in the loose refs directory, could prevent us from being able
to create the lockfile or delete the loose reference file.

The lock conflict scenario is the only one that really worries me.

But I don't think it is *so* hard to keep the current "best-effort"
behavior. I am working on a function delete_refs() for the reference API
that deletes a bunch of references on a "best effort" basis, reporting
per-reference errors for any deletions that fail. Longer term, we could
add something like a REFS_NON_ATOMIC flag to the
refs_transaction_update() functions, which allows the rest of the
transaction to proceed even if that particular update fails.

Michael

[1] I hope to submit a patch to make it possible to retry lock
acquisition with a specified timeout. This should help in scenarios like
this.

[2] Maybe a careful analysis or a slight change to our locking protocol
could prove that the only lock acquisition that can fail is the one on
packed-refs, which would cause all of the deletes to fail anyway. In
that case per-reference failures would cease to be a worry.

-- 
Michael Haggerty
mhagger@alum.mit.edu

Re: Why is "git fetch --prune" so much slower than "git remote prune"?

[Junio C Hamano]

Michael Haggerty <mhagger@alum.mit.edu> writes:

> The lock conflict scenario is the only one that really worries me.

Actually, I'd feel worried if this were on the receive-pack side, as
it is entirely valid that two or more people make uncoordinated push
into a single meeting point, but much less for something like
"fetch".  I do not see a valid reason why somebody cannot avoid
fetching into the same repository in a racy way.

> But I don't think it is *so* hard to keep the current "best-effort"
> behavior.

I agree that transaction is a wrong way to look at this.  I view "We
know packed refs is a lot more efficient if written once" as in the
same category as "We know we will register many new objects with
this operation, so instead of creating them into separate loose
object files, declare that the object store is 'plugged' and send
them into a single new packfile" (aka "bulk checkin").  Both are
uninterested in all-or-none execution, but merely want to declare a
boundary between a group of operations to help Git optimize them.

> I am working on a function delete_refs() for the reference API
> that deletes a bunch of references on a "best effort" basis, reporting
> per-reference errors for any deletions that fail.

I think it would make sense, but I suspect you would want to make it
clear that the function is best-effort-bulk somewhere in its name
(e.g. delete-refs-if-able or something) and make it not to
automatically fail the surrounding transaction if any.  That way
an application could do things like this:

    transaction-begin;
      create-ref;
      update-ref;
      ...
      delete-refs-if-able;
      if trying to be as atomic as possible
      then
	if previous delete-refs-if-able found undeletable refs
        then
          transaction-abort;
	fi
      fi
    transaction-commit;

Of course, supplying delete-refs that is not best-effort separately
would make it even easier to code the above (the last if/then/fi will
become unnecessary).