On Sat, Sep 29, 2018 at 07:36:08AM +0200, Duy Nguyen wrote:
> On Fri, Sep 28, 2018 at 06:24:58PM +0200, SZEDER Gábor wrote:
> > When unpack_trees() constructs a new index, it copies cache entries
> > from the original index [1]. prepare_to_write_split_index() has to
> > deal with this, and it has a dedicated code path for copied entries
> > that are present in the shared index, where it compares the cached
> > data in the corresponding copied and original entries. If the cached
> > data matches, then they are considered the same; if it differs, then
> > the copied entry will be marked for inclusion as a replacement entry
> > in the just about to be written split index by setting the
> > CE_UPDATE_IN_BASE flag.
> >
> > However, a cache entry already has its CE_UPDATE_IN_BASE flag set upon
> > reading the split index, if the entry already has a replacement entry
> > there, or upon refreshing the cached stat data, if the corresponding
> > file was modified. The state of this flag is then preserved when
> > unpack_trees() copies a cache entry from the shared index.
> >
> > So modify prepare_to_write_split_index() to check the copied cache
> > entries' CE_UPDATE_IN_BASE flag first, and skip the thorough
> > comparison of cached data if the flag is already set.
>
> OK so this is an optimization, not a bug fix. Right?
Well, a microoptimization at most: with all what's going on in
unpack_trees() I seriously doubt that it's effect is measurable.
> > Note that comparing the cached data in copied and original entries in
>
> s/cached data/cached stat data/ ? I was confused for a bit.
No, it's indeed cached data, but now that you mention it, the subject
line does need a s/stat //.
The comparison is done with this call:
ret = memcmp(&ce->ce_stat_data, &base->ce_stat_data,
offsetof(struct cache_entry, name) -
offsetof(struct cache_entry, ce_stat_data));
i.e. it starts at the stat data and ends just before the cache entry's
name, and 'struct cache_entry' has several other fields between these
two, including e.g. the cached oid:
struct cache_entry {
struct hashmap_entry ent;
struct stat_data ce_stat_data;
unsigned int ce_mode;
unsigned int ce_flags;
unsigned int mem_pool_allocated;
unsigned int ce_namelen;
unsigned int index; /* for link extension */
struct object_id oid;
char name[FLEX_ARRAY]; /* more */
};
However, to me it's mostly about clarity of the code, and about
documenting that CE_UPDATE_IN_BASE might have already been set at that
point and why, so the next dev diving in to debug the split index
doesn't have to figure this out himself.
> > the shared index might actually be entirely unnecessary. In theory
> > all code paths refreshing the cached stat data of an entry in the
> > shared index should set the CE_UPDATE_IN_BASE flag in that entry, and
> > unpack_trees() should preserve this flag when copying cache entries.
> > This means that the cached data is only ever changed if the
> > CE_UPDATE_IN_BASE flag is set as well. Our test suite seems to
> > confirm this: instrumenting the conditions in question and running the
> > test suite repeatedly with 'GIT_TEST_SPLIT_INDEX=yes' showed that the
> > cached data in a copied entry differs from the data in the shared
> > entry only if its CE_UPDATE_IN_BASE flag is indeed set.
>
> Yes I was probably just being paranoid (or sticking to simpler
> checks). I was told that split index is computation expensive and not
> doing unnecesary/expensive checks may help. But let's leave it for
> later.
>
> > + } else {
> > + /*
> > +* Thoroughly compare the cached data to see
> > +* whether it should be marked for inclusion
> > +* in the split index.
> > +*
> > +* This comparison might be unnecessary, as
> > +* code paths modifying the cached data do
> > +* set CE_UPDATE_IN_BASE as well.
> > +*/
> > + const unsigned int ondisk_flags =
> > + CE_STAGEMASK | CE_VALID |
> > + CE_EXTENDED_FLAGS;
> > + unsigned int ce_flags, base_flags, ret;
> > + ce_flags = ce->ce_flags;
> > + base_flags = base->ce_flags;
> > + /* only on-disk flags matter */
> > + ce->ce_flags &= ondisk_flags;
> > + base->ce_flags &= ondisk_flags;
> > + ret = memcmp(&ce->ce_stat_data,
> > &base->ce_stat_data,
> > +offsetof(struct cache_entry, name)
> > -
> > +
