Re: [PATCH - resend*2] VFS: use synchronize_rcu_expedited() in namespace_unlock()
On Thu, Nov 29 2018, Al Viro wrote:
> On Fri, Nov 30, 2018 at 10:33:18AM +1100, NeilBrown wrote:
>>
>> The synchronize_rcu() in namespace_unlock() is called every time
>> a filesystem is unmounted. If a great many filesystems are mounted,
>> this can cause a noticable slow-down in, for example, system shutdown.
>>
>> The sequence:
>> mkdir -p /tmp/Mtest/{0..5000}
>> time for i in /tmp/Mtest/*; do mount -t tmpfs tmpfs $i ; done
>> time umount /tmp/Mtest/*
>>
>> on a 4-cpu VM can report 8 seconds to mount the tmpfs filesystems, and
>> 100 seconds to unmount them.
>>
>> Boot the same VM with 1 CPU and it takes 18 seconds to mount the
>> tmpfs filesystems, but only 36 to unmount.
>>
>> If we change the synchronize_rcu() to synchronize_rcu_expedited()
>> the umount time on a 4-cpu VM drop to 0.6 seconds
>>
>> I think this 200-fold speed up is worth the slightly high system
>> impact of using synchronize_rcu_expedited().
>>
>> Acked-by: Paul E. McKenney (from general rcu
>> perspective)
>> Signed-off-by: NeilBrown
>> ---
>>
>> Al Viro says "I can live with this one" but this still hasn't landed.
>> Maybe someone else could apply it?
>
> Applied (in work.misc, once I push it out)
Excellent - thanks a lot :-)
NeilBrown
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Re: [PATCH - resend*2] VFS: use synchronize_rcu_expedited() in namespace_unlock()
On Thu, Nov 29 2018, Al Viro wrote:
> On Fri, Nov 30, 2018 at 10:33:18AM +1100, NeilBrown wrote:
>>
>> The synchronize_rcu() in namespace_unlock() is called every time
>> a filesystem is unmounted. If a great many filesystems are mounted,
>> this can cause a noticable slow-down in, for example, system shutdown.
>>
>> The sequence:
>> mkdir -p /tmp/Mtest/{0..5000}
>> time for i in /tmp/Mtest/*; do mount -t tmpfs tmpfs $i ; done
>> time umount /tmp/Mtest/*
>>
>> on a 4-cpu VM can report 8 seconds to mount the tmpfs filesystems, and
>> 100 seconds to unmount them.
>>
>> Boot the same VM with 1 CPU and it takes 18 seconds to mount the
>> tmpfs filesystems, but only 36 to unmount.
>>
>> If we change the synchronize_rcu() to synchronize_rcu_expedited()
>> the umount time on a 4-cpu VM drop to 0.6 seconds
>>
>> I think this 200-fold speed up is worth the slightly high system
>> impact of using synchronize_rcu_expedited().
>>
>> Acked-by: Paul E. McKenney (from general rcu
>> perspective)
>> Signed-off-by: NeilBrown
>> ---
>>
>> Al Viro says "I can live with this one" but this still hasn't landed.
>> Maybe someone else could apply it?
>
> Applied (in work.misc, once I push it out)
Excellent - thanks a lot :-)
NeilBrown
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Re: [PATCH - resend*2] VFS: use synchronize_rcu_expedited() in namespace_unlock()
On Fri, Nov 30, 2018 at 10:33:18AM +1100, NeilBrown wrote:
>
> The synchronize_rcu() in namespace_unlock() is called every time
> a filesystem is unmounted. If a great many filesystems are mounted,
> this can cause a noticable slow-down in, for example, system shutdown.
>
> The sequence:
> mkdir -p /tmp/Mtest/{0..5000}
> time for i in /tmp/Mtest/*; do mount -t tmpfs tmpfs $i ; done
> time umount /tmp/Mtest/*
>
> on a 4-cpu VM can report 8 seconds to mount the tmpfs filesystems, and
> 100 seconds to unmount them.
>
> Boot the same VM with 1 CPU and it takes 18 seconds to mount the
> tmpfs filesystems, but only 36 to unmount.
>
> If we change the synchronize_rcu() to synchronize_rcu_expedited()
> the umount time on a 4-cpu VM drop to 0.6 seconds
>
> I think this 200-fold speed up is worth the slightly high system
> impact of using synchronize_rcu_expedited().
>
> Acked-by: Paul E. McKenney (from general rcu
> perspective)
> Signed-off-by: NeilBrown
> ---
>
> Al Viro says "I can live with this one" but this still hasn't landed.
> Maybe someone else could apply it?
Applied (in work.misc, once I push it out)
Re: [PATCH - resend*2] VFS: use synchronize_rcu_expedited() in namespace_unlock()
On Fri, Nov 30, 2018 at 10:33:18AM +1100, NeilBrown wrote:
>
> The synchronize_rcu() in namespace_unlock() is called every time
> a filesystem is unmounted. If a great many filesystems are mounted,
> this can cause a noticable slow-down in, for example, system shutdown.
>
> The sequence:
> mkdir -p /tmp/Mtest/{0..5000}
> time for i in /tmp/Mtest/*; do mount -t tmpfs tmpfs $i ; done
> time umount /tmp/Mtest/*
>
> on a 4-cpu VM can report 8 seconds to mount the tmpfs filesystems, and
> 100 seconds to unmount them.
>
> Boot the same VM with 1 CPU and it takes 18 seconds to mount the
> tmpfs filesystems, but only 36 to unmount.
>
> If we change the synchronize_rcu() to synchronize_rcu_expedited()
> the umount time on a 4-cpu VM drop to 0.6 seconds
>
> I think this 200-fold speed up is worth the slightly high system
> impact of using synchronize_rcu_expedited().
>
> Acked-by: Paul E. McKenney (from general rcu
> perspective)
> Signed-off-by: NeilBrown
> ---
>
> Al Viro says "I can live with this one" but this still hasn't landed.
> Maybe someone else could apply it?
Applied (in work.misc, once I push it out)
Re: [PATCH - resend*2] VFS: use synchronize_rcu_expedited() in namespace_unlock()
The synchronize_rcu() in namespace_unlock() is called every time
a filesystem is unmounted. If a great many filesystems are mounted,
this can cause a noticable slow-down in, for example, system shutdown.
The sequence:
mkdir -p /tmp/Mtest/{0..5000}
time for i in /tmp/Mtest/*; do mount -t tmpfs tmpfs $i ; done
time umount /tmp/Mtest/*
on a 4-cpu VM can report 8 seconds to mount the tmpfs filesystems, and
100 seconds to unmount them.
Boot the same VM with 1 CPU and it takes 18 seconds to mount the
tmpfs filesystems, but only 36 to unmount.
If we change the synchronize_rcu() to synchronize_rcu_expedited()
the umount time on a 4-cpu VM drop to 0.6 seconds
I think this 200-fold speed up is worth the slightly high system
impact of using synchronize_rcu_expedited().
Acked-by: Paul E. McKenney (from general rcu
perspective)
Signed-off-by: NeilBrown
---
Al Viro says "I can live with this one" but this still hasn't landed.
Maybe someone else could apply it?
Thanks,
NeilBrown
Full quote from Al on 5th Oct:
> Umm... IIRC, the last one got sidetracked on the other thing in the series...
> that was s_anon stuff. I can live with this one; FWIW, what kind
> of load would trigger the impact of the change? Paul?
which Paul replied to.
fs/namespace.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/namespace.c b/fs/namespace.c
index a7f91265ea67..43a0d2c7449d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1360,7 +1360,7 @@ static void namespace_unlock(void)
if (likely(hlist_empty()))
return;
- synchronize_rcu();
+ synchronize_rcu_expedited();
group_pin_kill();
}
--
2.14.0.rc0.dirty
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Re: [PATCH - resend*2] VFS: use synchronize_rcu_expedited() in namespace_unlock()
The synchronize_rcu() in namespace_unlock() is called every time
a filesystem is unmounted. If a great many filesystems are mounted,
this can cause a noticable slow-down in, for example, system shutdown.
The sequence:
mkdir -p /tmp/Mtest/{0..5000}
time for i in /tmp/Mtest/*; do mount -t tmpfs tmpfs $i ; done
time umount /tmp/Mtest/*
on a 4-cpu VM can report 8 seconds to mount the tmpfs filesystems, and
100 seconds to unmount them.
Boot the same VM with 1 CPU and it takes 18 seconds to mount the
tmpfs filesystems, but only 36 to unmount.
If we change the synchronize_rcu() to synchronize_rcu_expedited()
the umount time on a 4-cpu VM drop to 0.6 seconds
I think this 200-fold speed up is worth the slightly high system
impact of using synchronize_rcu_expedited().
Acked-by: Paul E. McKenney (from general rcu
perspective)
Signed-off-by: NeilBrown
---
Al Viro says "I can live with this one" but this still hasn't landed.
Maybe someone else could apply it?
Thanks,
NeilBrown
Full quote from Al on 5th Oct:
> Umm... IIRC, the last one got sidetracked on the other thing in the series...
> that was s_anon stuff. I can live with this one; FWIW, what kind
> of load would trigger the impact of the change? Paul?
which Paul replied to.
fs/namespace.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/namespace.c b/fs/namespace.c
index a7f91265ea67..43a0d2c7449d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1360,7 +1360,7 @@ static void namespace_unlock(void)
if (likely(hlist_empty()))
return;
- synchronize_rcu();
+ synchronize_rcu_expedited();
group_pin_kill();
}
--
2.14.0.rc0.dirty
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