Re: [PATCH] t7063: work around FreeBSD's lazy mtime update feature

[Junio C Hamano]

Duy Nguyen  writes:

> OK how about this squashed in? The name was taken from fbsd definition
> IN_LAZYMOD.

I am sorry that I didn't spot this possiblity earlier, but do we
need anything conditional?  Either FREEBSD or LAZYMOD prerequisite
tells very little what the "Work around lazy mtime update" is and
where it triggers (I think the conclusion of your investigation was
that the timestamp on the containing directory, but that is ONLY
recorded in the log message, i.e. readers need to run 'git blame'
to find out).

It might be a better approach to have a helper function with
descriptive name and comment early in t7063, e.g.

# On some filesystems (e.g. FreeBSD's ext2 and ufs) this
# and that happens when we do blah, which forces the
# untracked cache code to take the slow path.  A test
# that wants to make sure the fast path works correctly
# should call this helper to make mtime of the containing
# directory in sync with the reality after doing blah and
# before checking the fast path behaviour
test_sync_directory_mtime () {
ls -ld . >/dev/null
}

and then call it at strategic places without any prerequisite.

The helper may turn out to be useful outside the context of 7063
later, at which time we can move it to test-lib-functions, but that
is a separate step.

> Off topic. Since I found this macro defined twice, in ext2 and ufs,
> but not in zfs (found its source!), I assume zfs does not have this
> particular feature (but I didn't test it). Untracked cache may be more
> effecient there.


> -- 8< --
> diff --git a/t/t7063-status-untracked-cache.sh 
> b/t/t7063-status-untracked-cache.sh
> index 08fc586..8bb048a 100755
> --- a/t/t7063-status-untracked-cache.sh
> +++ b/t/t7063-status-untracked-cache.sh
> @@ -419,7 +419,7 @@ test_expect_success 'create/modify files, some of which 
> are gitignored' '
>   rm base
>  '
>  
> -test_expect_success FREEBSD 'Work around lazy mtime update' '
> +test_expect_success LAZYMOD 'Work around lazy mtime update' '
>   ls -ld . >/dev/null
>  '
>  
> diff --git a/t/test-lib.sh b/t/test-lib.sh
> index 3c730a2..1fc5266 100644
> --- a/t/test-lib.sh
> +++ b/t/test-lib.sh
> @@ -962,7 +962,7 @@ case $(uname -s) in
>   test_set_prereq GREP_STRIPS_CR
>   ;;
>  *FreeBSD*)
> - test_set_prereq FREEBSD
> + test_set_prereq LAZYMOD
>   test_set_prereq POSIXPERM
>   test_set_prereq BSLASHPSPEC
>   test_set_prereq EXECKEEPSPID
> -- 8< --
>
> --
> Duy
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Re: [PATCH] t7063: work around FreeBSD's lazy mtime update feature

[Duy Nguyen]

On Mon, Aug 01, 2016 at 02:04:44PM -0700, Junio C Hamano wrote:
> Duy Nguyen  writes:
> 
> > On Mon, Aug 1, 2016 at 3:37 AM, Torstem Bögershausen  wrote:
> >> the term FREEBSD may be too generic to point out a single feature
> >> in an OS distributution.
> >> Following your investigations, it may even be possible that
> >> other systems adapt this "feature"?
> >>
> >> How about
> >> LAZY_DIR_MTIME_UPDATE
> >> (or similar)
> >
> > This feature was added in 1998, so yes there's a chance it has spread
> > to a few fbsd derivatives (not sure if openbsd or netbsd is close
> > enough and they ever exchange changes). But I'd rather wait for the
> > second OS to expose the same feature before renaming it.
> 
> I think a name based on the observed behaviour ("feature") would be
> more appropriate because I'd be more worried about us finding other
> glitches we see (initially) only on FBSD.  People who need to adjust
> tests that use the same FBSD prereq would have to wonder which
> prereq-skip is due to which glitch.

OK how about this squashed in? The name was taken from fbsd definition
IN_LAZYMOD.

Off topic. Since I found this macro defined twice, in ext2 and ufs,
but not in zfs (found its source!), I assume zfs does not have this
particular feature (but I didn't test it). Untracked cache may be more
effecient there.

-- 8< --
diff --git a/t/t7063-status-untracked-cache.sh 
b/t/t7063-status-untracked-cache.sh
index 08fc586..8bb048a 100755
--- a/t/t7063-status-untracked-cache.sh
+++ b/t/t7063-status-untracked-cache.sh
@@ -419,7 +419,7 @@ test_expect_success 'create/modify files, some of which are 
gitignored' '
rm base
 '
 
-test_expect_success FREEBSD 'Work around lazy mtime update' '
+test_expect_success LAZYMOD 'Work around lazy mtime update' '
ls -ld . >/dev/null
 '
 
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 3c730a2..1fc5266 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -962,7 +962,7 @@ case $(uname -s) in
test_set_prereq GREP_STRIPS_CR
;;
 *FreeBSD*)
-   test_set_prereq FREEBSD
+   test_set_prereq LAZYMOD
test_set_prereq POSIXPERM
test_set_prereq BSLASHPSPEC
test_set_prereq EXECKEEPSPID
-- 8< --

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Re: [PATCH] t7063: work around FreeBSD's lazy mtime update feature

[Junio C Hamano]

Duy Nguyen  writes:

> On Mon, Aug 1, 2016 at 3:37 AM, Torstem Bögershausen  wrote:
>> the term FREEBSD may be too generic to point out a single feature
>> in an OS distributution.
>> Following your investigations, it may even be possible that
>> other systems adapt this "feature"?
>>
>> How about
>> LAZY_DIR_MTIME_UPDATE
>> (or similar)
>
> This feature was added in 1998, so yes there's a chance it has spread
> to a few fbsd derivatives (not sure if openbsd or netbsd is close
> enough and they ever exchange changes). But I'd rather wait for the
> second OS to expose the same feature before renaming it.

I think a name based on the observed behaviour ("feature") would be
more appropriate because I'd be more worried about us finding other
glitches we see (initially) only on FBSD.  People who need to adjust
tests that use the same FBSD prereq would have to wonder which
prereq-skip is due to which glitch.
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Re: [PATCH] t7063: work around FreeBSD's lazy mtime update feature

[Duy Nguyen]

On Mon, Aug 1, 2016 at 3:37 AM, Torstem Bögershausen  wrote:
> the term FREEBSD may be too generic to point out a single feature
> in an OS distributution.
> Following your investigations, it may even be possible that
> other systems adapt this "feature"?
>
> How about
> LAZY_DIR_MTIME_UPDATE
> (or similar)

This feature was added in 1998, so yes there's a chance it has spread
to a few fbsd derivatives (not sure if openbsd or netbsd is close
enough and they ever exchange changes). But I'd rather wait for the
second OS to expose the same feature before renaming it.
-- 
Duy
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Re: [PATCH] t7063: work around FreeBSD's lazy mtime update feature

[Torstem Bögershausen]

> Am 30.07.2016 um 15:20 schrieb Nguyễn Thái Ngọc Duy :
> 
> Let's start with the commit message of [1] from freebsd.git [2]
> 
>Sync timestamp changes for inodes of special files to disk as late
>as possible (when the inode is reclaimed).  Temporarily only do
>this if option UFS_LAZYMOD configured and softupdates aren't
>enabled.  UFS_LAZYMOD is intentionally left out of
>/sys/conf/options.
> 
>This is mainly to avoid almost useless disk i/o on battery powered
>machines.  It's silly to write to disk (on the next sync or when
>the inode becomes inactive) just because someone hit a key or
>something wrote to the screen or /dev/null.
> 
>PR: 5577 [3]
> 
> The short version of that, in the context of t7063, is that when a
> directory is updated, its mtime may be updated later, not
> immediately. This can be shown with a simple command sequence
> 
>date; sleep 1; touch abc; rm abc; sleep 10; ls -lTd .
> 
> One would expect that the date shown in `ls` would be one second from
> `date`, but it's 10 seconds later. If we put another `ls -lTd .` in
> front of `sleep 10`, then the date of the last `ls` comes as
> expected. The first `ls` somehow forces mtime to be updated.
> 
> t7063 is really sensitive to directory mtime. When mtime is too "new",
> git code suspects racy timestamps and will not trigger the shortcut in
> untracked cache, in t7063.26 (or t7063.25 before this patch) and
> eventually be detected in t7063.28
> 
> We have two options thanks to this special FreeBSD feature:
> 
> 1) Stop supporting untracked cache on FreeBSD. Skip t7063 entirely
>   when running on FreeBSD
> 
> 2) Work around this problem (using the same 'ls' trick) and continue
>   to support untracked cache on FreeBSD
> 
> I initially wanted to go with 1) because I didn't know the exact
> nature of this feature and feared that it would make untracked cache
> work unreliably, using the cached version when it should not.
> 
> Since the behavior of this thing is clearer now. The picture is not
> that bad. If this indeed happens often, untracked cache would assume
> racy condition more often and _fall back_ to non-untracked cache code
> paths. Which means it may be less effective, but it will not show
> wrong things.
> 
> This patch goes with option 2.
> 
> PS. For those who want to look further in FreeBSD source code, this
> flag is now called IN_LAZYMOD. I can see it's effective in ext2 and
> ufs. zfs is questionable.
> 
> [1] 660e6408e6df99a20dacb070c5e7f9739efdf96d
> [2] git://github.com/freebsd/freebsd.git
> [3] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=5577
> 
> Reported-by: Eric Wong 
> Signed-off-by: Nguyễn Thái Ngọc Duy 
> ---
> This is only of those commits that commit messages are more important
> than the patch itself. One of the good notes about directory mtime,
> if we start to use it in more places in git.
> 
> t/t7063-status-untracked-cache.sh | 4 
> t/test-lib.sh | 6 ++
> 2 files changed, 10 insertions(+)
> 
> diff --git a/t/t7063-status-untracked-cache.sh 
> b/t/t7063-status-untracked-cache.sh
> index a971884..08fc586 100755
> --- a/t/t7063-status-untracked-cache.sh
> +++ b/t/t7063-status-untracked-cache.sh
> @@ -419,6 +419,10 @@ test_expect_success 'create/modify files, some of which 
> are gitignored' '
>rm base
> '
> 
> +test_expect_success FREEBSD 'Work around lazy mtime update' '
> +ls -ld . >/dev/null
> +'
> +

the term FREEBSD may be too generic to point out a single feature
in an OS distributution.
Following your investigations, it may even be possible that
other systems adapt this "feature"?

How about
LAZY_DIR_MTIME_UPDATE
(or similar)

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Re: [PATCH] t7063: work around FreeBSD's lazy mtime update feature

[Duy Nguyen]

On Sun, Jul 31, 2016 at 3:07 AM, Eric Wong  wrote:
>> would be more to the point of what is going on, here.   But I
>> also wonder if untracked cache itself could/should be doing this
>> internally.
>
> Still wondering :>

There's nothing we can do besides maybe run a cron job executing
'sync' every second or so. We need to force mtime to be written down
close to.. you know.. mtime. By the time git is executed, it's already
late. You can execute 'sync' inside git then wait a couple seconds..
but that's just stupid. And removing the racy check is even more
dangerous, now you can get false output.
-- 
Duy
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Re: [PATCH] t7063: work around FreeBSD's lazy mtime update feature

[Eric Wong]

Eric Wong  wrote:
> Nguyễn Thái Ngọc Duy  wrote:
> > +test_expect_success FREEBSD 'Work around lazy mtime update' '
> > +   ls -ld . >/dev/null
> > +'
> 
>   stat . >/dev/null

If there's some older FreeBSD w/o stat(1); "test -x ."
ought to work, too, and it's faster being a shell builtin.

I suspect some shell might be clever about optimizing away
a more-obvious "test -d .", so I choose "test -x ."

> would be more to the point of what is going on, here.   But I
> also wonder if untracked cache itself could/should be doing this
> internally.

Still wondering :>

> (I'm not familiar with that code, of course)
> 
> Thanks again for looking into this.
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Re: [PATCH] t7063: work around FreeBSD's lazy mtime update feature

[Eric Wong]

Nguyễn Thái Ngọc Duy  wrote:
> Let's start with the commit message of [1] from freebsd.git [2]
> 
> Sync timestamp changes for inodes of special files to disk as late
> as possible (when the inode is reclaimed).  Temporarily only do
> this if option UFS_LAZYMOD configured and softupdates aren't
> enabled.  UFS_LAZYMOD is intentionally left out of
> /sys/conf/options.
> 
> This is mainly to avoid almost useless disk i/o on battery powered
> machines.  It's silly to write to disk (on the next sync or when
> the inode becomes inactive) just because someone hit a key or
> something wrote to the screen or /dev/null.
> 
> PR: 5577 [3]
> 
> The short version of that, in the context of t7063, is that when a
> directory is updated, its mtime may be updated later, not
> immediately. This can be shown with a simple command sequence
> 
> date; sleep 1; touch abc; rm abc; sleep 10; ls -lTd .

Yikes.  I guess FreeBSD doesn't have an in-memory inode cache it
can keep up-to-date without flushing to disk?

> One would expect that the date shown in `ls` would be one second from
> `date`, but it's 10 seconds later. If we put another `ls -lTd .` in
> front of `sleep 10`, then the date of the last `ls` comes as
> expected. The first `ls` somehow forces mtime to be updated.

Fwiw, `stat .` seems to have the same effect as `ls -lTd .`...

> t7063 is really sensitive to directory mtime. When mtime is too "new",
> git code suspects racy timestamps and will not trigger the shortcut in
> untracked cache, in t7063.26 (or t7063.25 before this patch) and
> eventually be detected in t7063.28
> 
> We have two options thanks to this special FreeBSD feature:
> 
> 1) Stop supporting untracked cache on FreeBSD. Skip t7063 entirely
>when running on FreeBSD
> 
> 2) Work around this problem (using the same 'ls' trick) and continue
>to support untracked cache on FreeBSD
> 
> I initially wanted to go with 1) because I didn't know the exact
> nature of this feature and feared that it would make untracked cache
> work unreliably, using the cached version when it should not.
> 
> Since the behavior of this thing is clearer now. The picture is not
> that bad. If this indeed happens often, untracked cache would assume
> racy condition more often and _fall back_ to non-untracked cache code
> paths. Which means it may be less effective, but it will not show
> wrong things.
> 
> This patch goes with option 2.
> 
> PS. For those who want to look further in FreeBSD source code, this
> flag is now called IN_LAZYMOD. I can see it's effective in ext2 and
> ufs. zfs is questionable.
> 
> [1] 660e6408e6df99a20dacb070c5e7f9739efdf96d
> [2] git://github.com/freebsd/freebsd.git
> [3] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=5577
> 
> Reported-by: Eric Wong 
> Signed-off-by: Nguyễn Thái Ngọc Duy 
> ---
>  This is only of those commits that commit messages are more important
>  than the patch itself. One of the good notes about directory mtime,
>  if we start to use it in more places in git.

Thanks, Tested-by: Eric Wong 

>  t/t7063-status-untracked-cache.sh | 4 
>  t/test-lib.sh | 6 ++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/t/t7063-status-untracked-cache.sh 
> b/t/t7063-status-untracked-cache.sh
> index a971884..08fc586 100755
> --- a/t/t7063-status-untracked-cache.sh
> +++ b/t/t7063-status-untracked-cache.sh
> @@ -419,6 +419,10 @@ test_expect_success 'create/modify files, some of which 
> are gitignored' '
>   rm base
>  '
>  
> +test_expect_success FREEBSD 'Work around lazy mtime update' '
> + ls -ld . >/dev/null
> +'

stat . >/dev/null

would be more to the point of what is going on, here.   But I
also wonder if untracked cache itself could/should be doing this
internally.

(I'm not familiar with that code, of course)

Thanks again for looking into this.
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[PATCH] t7063: work around FreeBSD's lazy mtime update feature

[Nguyễn Thái Ngọc Duy]

Let's start with the commit message of [1] from freebsd.git [2]

Sync timestamp changes for inodes of special files to disk as late
as possible (when the inode is reclaimed).  Temporarily only do
this if option UFS_LAZYMOD configured and softupdates aren't
enabled.  UFS_LAZYMOD is intentionally left out of
/sys/conf/options.

This is mainly to avoid almost useless disk i/o on battery powered
machines.  It's silly to write to disk (on the next sync or when
the inode becomes inactive) just because someone hit a key or
something wrote to the screen or /dev/null.

PR: 5577 [3]

The short version of that, in the context of t7063, is that when a
directory is updated, its mtime may be updated later, not
immediately. This can be shown with a simple command sequence

date; sleep 1; touch abc; rm abc; sleep 10; ls -lTd .

One would expect that the date shown in `ls` would be one second from
`date`, but it's 10 seconds later. If we put another `ls -lTd .` in
front of `sleep 10`, then the date of the last `ls` comes as
expected. The first `ls` somehow forces mtime to be updated.

t7063 is really sensitive to directory mtime. When mtime is too "new",
git code suspects racy timestamps and will not trigger the shortcut in
untracked cache, in t7063.26 (or t7063.25 before this patch) and
eventually be detected in t7063.28

We have two options thanks to this special FreeBSD feature:

1) Stop supporting untracked cache on FreeBSD. Skip t7063 entirely
   when running on FreeBSD

2) Work around this problem (using the same 'ls' trick) and continue
   to support untracked cache on FreeBSD

I initially wanted to go with 1) because I didn't know the exact
nature of this feature and feared that it would make untracked cache
work unreliably, using the cached version when it should not.

Since the behavior of this thing is clearer now. The picture is not
that bad. If this indeed happens often, untracked cache would assume
racy condition more often and _fall back_ to non-untracked cache code
paths. Which means it may be less effective, but it will not show
wrong things.

This patch goes with option 2.

PS. For those who want to look further in FreeBSD source code, this
flag is now called IN_LAZYMOD. I can see it's effective in ext2 and
ufs. zfs is questionable.

[1] 660e6408e6df99a20dacb070c5e7f9739efdf96d
[2] git://github.com/freebsd/freebsd.git
[3] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=5577

Reported-by: Eric Wong 
Signed-off-by: Nguyễn Thái Ngọc Duy 
---
 This is only of those commits that commit messages are more important
 than the patch itself. One of the good notes about directory mtime,
 if we start to use it in more places in git.

 t/t7063-status-untracked-cache.sh | 4 
 t/test-lib.sh | 6 ++
 2 files changed, 10 insertions(+)

diff --git a/t/t7063-status-untracked-cache.sh 
b/t/t7063-status-untracked-cache.sh
index a971884..08fc586 100755
--- a/t/t7063-status-untracked-cache.sh
+++ b/t/t7063-status-untracked-cache.sh
@@ -419,6 +419,10 @@ test_expect_success 'create/modify files, some of which 
are gitignored' '
rm base
 '
 
+test_expect_success FREEBSD 'Work around lazy mtime update' '
+   ls -ld . >/dev/null
+'
+
 test_expect_success 'test sparse status with untracked cache' '
: >../trace &&
avoid_racy &&
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 0055ebb..3c730a2 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -961,6 +961,12 @@ case $(uname -s) in
test_set_prereq SED_STRIPS_CR
test_set_prereq GREP_STRIPS_CR
;;
+*FreeBSD*)
+   test_set_prereq FREEBSD
+   test_set_prereq POSIXPERM
+   test_set_prereq BSLASHPSPEC
+   test_set_prereq EXECKEEPSPID
+   ;;
 *)
test_set_prereq POSIXPERM
test_set_prereq BSLASHPSPEC
-- 
2.9.1.566.gbd532d4

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