RE: [PATCH v1 2/2] exfat: hint the empty entry which at the end of cluster chain

Yuezhang.Mo
2022-11-01T03:15:42Z

At this stage, we can check this case in exfat_search_empty_slot() by removing the following if statement.

- if (num_entries <= hint_femp->count) {
- hint_femp->eidx = EXFAT_HINT_NONE;
- return dentry;
- }

What do you think?

> -----Original Message-----
> From: Mo, Yuezhang
> Sent: Monday, October 31, 2022 7:05 PM
> To: Namjae Jeon <linkinjeon@kernel.org>; Sungjong Seo
> <sj1557.seo@samsung.com>
> Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>; linux-kernel
> <linux-kernel@vger.kernel.org>
> Subject: RE: [PATCH v1 2/2] exfat: hint the empty entry which at the end of
> cluster chain
>
> > > This seems like a very good approach. Perhaps the key fix that
> > > improved performance seems to be the handling of cases where empty
> > > space was not found and ended with TYPE_UNUSED.
> > >
> > > However, there are concerns about trusting and using the number of
> > > free entries after TYPE_UNUSED calculated based on directory size.
> > > This is because, unlike exFAT Spec., in the real world, unexpected
> > > TYPE_UNUSED entries may exist. :( That's why
> > > exfat_search_empty_slot() checks if there is any valid entry after
> > > TYPE_UNUSED. In my experience, they can be caused by a wrong FS
> > > driver and H/W defects, and the probability of occurrence is not low.
> > >
> > > Therefore, when the lookup ends with TYPE_UNUSED, if there are no
> > > empty entries found yet, it would be better to set the last empty
> > > entry to hint_femp.eidx and set hint_femp.count to 0.
> > > If so, even if the lookup ends with TYPE_UNUSED,
> > > exfat_search_empty_slot() can start searching from the position of
> > > the last empty entry and check whether there are actually empty
> > > entries as many as the required num_entries as now.
> > >
> > > what do you think?
>
> We plan to add a new helper exfat_get_empty_dentry_set(), this helper is
> called before setting the entry type, it caches and then checks for empty
> entries(Check-on-write is safer than checking when looking for empty directory
> entries).
>
> for (i = 0; i < es->num_entries; i++) {
> ep = exfat_get_dentry_cached(es, i);
> type = exfat_get_entry_type(ep);
> if (type == TYPE_UNUSED)
> unused_hit = true;
> else if (type == TYPE_DELETED) {
> if (unused_hit)
> goto error;
> } else
> goto error;
> }
>
> This code is not ready, we are testing and internal reviewing.
>
> > -----Original Message-----
> > From: Namjae Jeon <linkinjeon@kernel.org>
> > Sent: Monday, October 31, 2022 2:32 PM
> > To: Sungjong Seo <sj1557.seo@samsung.com>; Mo, Yuezhang
> > <Yuezhang.Mo@sony.com>
> > Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>; linux-kernel
> > <linux-kernel@vger.kernel.org>
> > Subject: Re: [PATCH v1 2/2] exfat: hint the empty entry which at the
> > end of cluster chain
> >
> > Add missing Cc: Yuezhang Mo.
> >
> > 2022-10-31 15:17 GMT+09:00, Sungjong Seo <sj1557.seo@samsung.com>:
> > > Hi, Yuezhang Mo,
> > >
> > >> After traversing all directory entries, hint the empty directory
> > >> entry no matter whether or not there are enough empty directory
> > >> entries.
> > >>
> > >> After this commit, hint the empty directory entries like this:
> > >>
> > >> 1. Hint the deleted directory entries if enough; 2. Hint the
> > >> deleted and unused directory entries which at the
> > >> end of the cluster chain no matter whether enough or not(Add
> > >> by this commit);
> > >> 3. If no any empty directory entries, hint the empty directory
> > >> entries in the new cluster(Add by this commit).
> > >>
> > >> This avoids repeated traversal of directory entries, reduces CPU
> > >> usage, and improves the performance of creating files and
> > >> directories(especially on low-performance CPUs).
> > >>
> > >> Test create 5000 files in a class 4 SD card on imx6q-sabrelite
> > >> with:
> > >>
> > >> for ((i=0;i<5;i++)); do
> > >> sync
> > >> time (for ((j=1;j<=1000;j++)); do touch file$((i*1000+j)); done)
> > >> done
> > >>
> > >> The more files, the more performance improvements.
> > >>
> > >> Before After Improvement
> > >> 1~1000 25.360s 22.168s 14.40%
> > >> 1001~2000 38.242s 28.72ss 33.15%
> > >> 2001~3000 49.134s 35.037s 40.23%
> > >> 3001~4000 62.042s 41.624s 49.05%
> > >> 4001~5000 73.629s 46.772s 57.42%
> > >>
> > >> Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
> > >> Reviewed-by: Andy Wu <Andy.Wu@sony.com>
> > >> Reviewed-by: Aoyama Wataru <wataru.aoyama@sony.com>
> > >> ---
> > >> fs/exfat/dir.c | 26 ++++++++++++++++++++++----
> > >> fs/exfat/namei.c | 22 ++++++++++++++--------
> > >> 2 files changed, 36 insertions(+), 12 deletions(-)
> > >>
> > >> diff --git a/fs/exfat/dir.c b/fs/exfat/dir.c index
> > >> a569f285f4fd..7600f3521246 100644
> > >> --- a/fs/exfat/dir.c
> > >> +++ b/fs/exfat/dir.c
> > >> @@ -936,18 +936,25 @@ struct exfat_entry_set_cache
> > >> *exfat_get_dentry_set(struct super_block *sb,
> > >>
> > >> static inline void exfat_hint_empty_entry(struct exfat_inode_info *ei,
> > >> struct exfat_hint_femp *candi_empty, struct exfat_chain *clu,
> > >> - int dentry, int num_entries)
> > >> + int dentry, int num_entries, int entry_type)
> > >> {
> > >> if (ei->hint_femp.eidx == EXFAT_HINT_NONE ||
> > >> ei->hint_femp.count < num_entries ||
> > >> ei->hint_femp.eidx > dentry) {
> > >> + int total_entries = EXFAT_B_TO_DEN(i_size_read(&ei-
> > >> >vfs_inode));
> > >> +
> > >> if (candi_empty->count == 0) {
> > >> candi_empty->cur = *clu;
> > >> candi_empty->eidx = dentry;
> > >> }
> > >>
> > >> - candi_empty->count++;
> > >> - if (candi_empty->count == num_entries)
> > >> + if (entry_type == TYPE_UNUSED)
> > >> + candi_empty->count += total_entries - dentry;
> > >
> > > This seems like a very good approach. Perhaps the key fix that
> > > improved performance seems to be the handling of cases where empty
> > > space was not found and ended with TYPE_UNUSED.
> > >
> > > However, there are concerns about trusting and using the number of
> > > free entries after TYPE_UNUSED calculated based on directory size.
> > > This is because, unlike exFAT Spec., in the real world, unexpected
> > > TYPE_UNUSED entries may exist. :( That's why
> > > exfat_search_empty_slot() checks if there is any valid entry after
> > > TYPE_UNUSED. In my experience, they can be caused by a wrong FS
> > > driver and H/W defects, and the probability of occurrence is not low.
> > >
> > > Therefore, when the lookup ends with TYPE_UNUSED, if there are no
> > > empty entries found yet, it would be better to set the last empty
> > > entry to hint_femp.eidx and set hint_femp.count to 0.
> > > If so, even if the lookup ends with TYPE_UNUSED,
> > > exfat_search_empty_slot() can start searching from the position of
> > > the last empty entry and check whether there are actually empty
> > > entries as many as the required num_entries as now.
> > >
> > > what do you think?
> > >
> > >> + else
> > >> + candi_empty->count++;
> > >> +
> > >> + if (candi_empty->count == num_entries ||
> > >> + candi_empty->count + candi_empty->eidx == total_entries)
> > >> ei->hint_femp = *candi_empty;
> > >> }
> > >> }
> > > [snip]
> > >> --
> > >> 2.25.1
> > >
> > >