Re: [PATCH v2] send-pack: never fetch when checking exclusions
On Fri, Oct 11, 2019 at 03:08:22PM -0700, Jonathan Tan wrote:
> > As a general rule (and why I'm raising this issue in reply to Jonathan's
> > patch), I think most or all sites that want OBJECT_INFO_QUICK will want
> > SKIP_FETCH_OBJECT as well, and vice versa. The reasoning is generally
> > the same:
> >
> > - it's OK to racily have a false negative (we'll still be correct, but
> > possibly a little less optimal)
> >
> > - it's expected and normal to be missing the object, so spending time
> > double-checking the pack store wastes measurable time in real-world
> > cases
>
> I took a look on "next" and it's true for these reasons in most cases
> but not all.
Thanks for digging into this.
> QUICK implies SKIP_FETCH_OBJECT:
>
> fetch-pack.c: Run with fetch_if_missing=0 (from builtin/fetch.c,
> builtin/fetch-pack.c, or through a lazy fetch) so OK.
>
> builtin/index-pack.c: Run with fetch_if_missing=0, so OK.
>
> builtin/fetch.c: Run with fetch_if_missing=0, so OK.
>
> object-store.h, sha1-file.c: Definition and implementation of this
> flag.
Right, I think going in this direction is pretty simple. Having been
marked with QUICK, they hit both of my points from above. And if we want
to avoid re-scanning the pack directory because of cost, we _definitely_
want to avoid making an expensive network call.
> Everything is OK here. Now, SKIP_FETCH_OBJECT implies QUICK:
>
> cache-tree.c: I added this recently in f981ec18cf ("cache-tree: do not
> lazy-fetch tentative tree", 2019-09-09). No problem with a false
> negative, since we know how to reconstruct the tree. OK.
> [...]
> send-pack.c: This patch (which is already in "next"). If we have a
> false negative, we might accidentally send more than we need. But that
> is not too bad.
Yeah, I think both of these could be QUICK.
> promisor-remote.c: This is the slightly tricky one. We use this
> information to determine if we got our lazily-fetched object from the
> most recent lazy fetch, or if we should continue attempting to fetch the
> given object from other promisor remotes; so this information is
> important. However, adding QUICK doesn't lose us anything because the
> lack of QUICK only helps us when there is another process packing
> loose objects: if we got our object, our object will be in a pack
> (because of the way the fetch is implemented - in particular, we need
> a pack because we need the ".promisor" file).
>
> So everything is OK except for promisor-remote.c, but even that is OK
> for another reason.
Yeah, though I wouldn't be sad to see that use a separate flag, since it
really is about promisor logic.
That implies to me maybe we should be using QUICK more aggressively, and
QUICK should auto-imply SKIP_FETCH_OBJECT.
> Having said that, perhaps we should consider promisor-remote.c as
> low-level code and expect it to know that objects are fetched into a
> packfile (as opposed to loose objects), so it can safely use QUICK
> (which is documented as checking packed after packed and loose). If no
> one disagrees, I can make such a patch after jt/push-avoid-lazy-fetch is
> merged to master (as is the plan, according to What's Cooking [1]).
I think it's OK to continue leaving out QUICK there if it's not causing
problems. It really is a bit different than the other cases.
-Peff
Re: [PATCH v2] send-pack: never fetch when checking exclusions
Jeff King writes:
> As a general rule (and why I'm raising this issue in reply to Jonathan's
> patch), I think most or all sites that want OBJECT_INFO_QUICK will want
> SKIP_FETCH_OBJECT as well, and vice versa. The reasoning is generally
> the same:
>
> - it's OK to racily have a false negative (we'll still be correct, but
> possibly a little less optimal)
>
> - it's expected and normal to be missing the object, so spending time
> double-checking the pack store wastes measurable time in real-world
> cases
31f5256c ("sha1-file: split OBJECT_INFO_FOR_PREFETCH", 2019-05-28)
separated SKIP_FETCH_OBJECT out of FOR_PREFETCH, the latter of which
was and is SKIP_FETCH and QUICK combined. Use SKIP_FETCH_OBJECT
alone may need to be re-examined and discouraged?
Re: [PATCH v2] send-pack: never fetch when checking exclusions
> As a general rule (and why I'm raising this issue in reply to Jonathan's
> patch), I think most or all sites that want OBJECT_INFO_QUICK will want
> SKIP_FETCH_OBJECT as well, and vice versa. The reasoning is generally
> the same:
>
> - it's OK to racily have a false negative (we'll still be correct, but
> possibly a little less optimal)
>
> - it's expected and normal to be missing the object, so spending time
> double-checking the pack store wastes measurable time in real-world
> cases
I took a look on "next" and it's true for these reasons in most cases
but not all.
QUICK implies SKIP_FETCH_OBJECT:
fetch-pack.c: Run with fetch_if_missing=0 (from builtin/fetch.c,
builtin/fetch-pack.c, or through a lazy fetch) so OK.
builtin/index-pack.c: Run with fetch_if_missing=0, so OK.
builtin/fetch.c: Run with fetch_if_missing=0, so OK.
object-store.h, sha1-file.c: Definition and implementation of this
flag.
Everything is OK here. Now, SKIP_FETCH_OBJECT implies QUICK:
cache-tree.c: I added this recently in f981ec18cf ("cache-tree: do not
lazy-fetch tentative tree", 2019-09-09). No problem with a false
negative, since we know how to reconstruct the tree. OK.
object-store.h, sha1-file.c: Definition and implementation of this
flag.
send-pack.c: This patch (which is already in "next"). If we have a
false negative, we might accidentally send more than we need. But that
is not too bad.
promisor-remote.c: This is the slightly tricky one. We use this
information to determine if we got our lazily-fetched object from the
most recent lazy fetch, or if we should continue attempting to fetch the
given object from other promisor remotes; so this information is
important. However, adding QUICK doesn't lose us anything because the
lack of QUICK only helps us when there is another process packing
loose objects: if we got our object, our object will be in a pack
(because of the way the fetch is implemented - in particular, we need
a pack because we need the ".promisor" file).
So everything is OK except for promisor-remote.c, but even that is OK
for another reason.
Having said that, perhaps we should consider promisor-remote.c as
low-level code and expect it to know that objects are fetched into a
packfile (as opposed to loose objects), so it can safely use QUICK
(which is documented as checking packed after packed and loose). If no
one disagrees, I can make such a patch after jt/push-avoid-lazy-fetch is
merged to master (as is the plan, according to What's Cooking [1]).
[1] https://public-inbox.org/git/xmqq8sprhgzc@gitster-ct.c.googlers.com/
Re: [PATCH v2] send-pack: never fetch when checking exclusions
On Fri, Oct 11, 2019 at 08:31:30AM -0400, Derrick Stolee wrote: > >> Ensure that these lazy fetches do not occur. > > > > That makes sense. For similar reasons, should we be using > > OBJECT_INFO_QUICK here? If the other side has a bunch of ref tips that > > we don't have, we'll end up re-scanning the pack directory over and over > > (which is _usually_ pretty quick, but can be slow if you have a lot of > > packs locally). And it's OK if we racily miss out on an exclusion due to > > somebody else repacking simultaneously. > > That's a good idea. We can hint to the object store that we don't expect > misses to be due to a concurrent repack, so we don't want to reprepare > pack-files. As a general rule (and why I'm raising this issue in reply to Jonathan's patch), I think most or all sites that want OBJECT_INFO_QUICK will want SKIP_FETCH_OBJECT as well, and vice versa. The reasoning is generally the same: - it's OK to racily have a false negative (we'll still be correct, but possibly a little less optimal) - it's expected and normal to be missing the object, so spending time double-checking the pack store wastes measurable time in real-world cases -Peff
Re: [PATCH v2] send-pack: never fetch when checking exclusions
On 10/11/2019 2:12 AM, Jeff King wrote: > On Tue, Oct 08, 2019 at 11:37:39AM -0700, Jonathan Tan wrote: > >> When building the packfile to be sent, send_pack() is given a list of >> remote refs to be used as exclusions. For each ref, it first checks if >> the ref exists locally, and if it does, passes it with a "^" prefix to >> pack-objects. However, in a partial clone, the check may trigger a lazy >> fetch. >> >> The additional commit ancestry information obtained during such fetches >> may show that certain objects that would have been sent are already >> known to the server, resulting in a smaller pack being sent. But this is >> at the cost of fetching from many possibly unrelated refs, and the lazy >> fetches do not help at all in the typical case where the client is >> up-to-date with the upstream of the branch being pushed. >> >> Ensure that these lazy fetches do not occur. > > That makes sense. For similar reasons, should we be using > OBJECT_INFO_QUICK here? If the other side has a bunch of ref tips that > we don't have, we'll end up re-scanning the pack directory over and over > (which is _usually_ pretty quick, but can be slow if you have a lot of > packs locally). And it's OK if we racily miss out on an exclusion due to > somebody else repacking simultaneously. That's a good idea. We can hint to the object store that we don't expect misses to be due to a concurrent repack, so we don't want to reprepare pack-files. -Stolee
Re: [PATCH v2] send-pack: never fetch when checking exclusions
On Tue, Oct 08, 2019 at 11:37:39AM -0700, Jonathan Tan wrote: > When building the packfile to be sent, send_pack() is given a list of > remote refs to be used as exclusions. For each ref, it first checks if > the ref exists locally, and if it does, passes it with a "^" prefix to > pack-objects. However, in a partial clone, the check may trigger a lazy > fetch. > > The additional commit ancestry information obtained during such fetches > may show that certain objects that would have been sent are already > known to the server, resulting in a smaller pack being sent. But this is > at the cost of fetching from many possibly unrelated refs, and the lazy > fetches do not help at all in the typical case where the client is > up-to-date with the upstream of the branch being pushed. > > Ensure that these lazy fetches do not occur. That makes sense. For similar reasons, should we be using OBJECT_INFO_QUICK here? If the other side has a bunch of ref tips that we don't have, we'll end up re-scanning the pack directory over and over (which is _usually_ pretty quick, but can be slow if you have a lot of packs locally). And it's OK if we racily miss out on an exclusion due to somebody else repacking simultaneously. -Peff
