On Oct 25, 2016, at 4:44 PM, Omar Sandoval <osan...@osandov.com> wrote:
>
> On Tue, Oct 25, 2016 at 02:41:44PM -0400, Josef Bacik wrote:
>> With anything that populates the inode/dentry cache with a lot of one time
>> use
>> inodes we can really put a lot of pressure on the system for things we don't
>> need to keep in cache. It takes two runs through the LRU to evict these one
>> use
>> entries, and if you have a lot of memory you can end up with 10's of
>> millions of
>> entries in the dcache or icache that have never actually been touched since
>> they
>> were first instantiated, and it will take a lot of CPU and a lot of pressure
>> to
>> evict all of them.
>>
>> So instead do what we do with pagecache, only set the *REFERENCED flags if we
>> are being used after we've been put onto the LRU. This makes a significant
>> difference in the system's ability to evict these useless cache entries.
>> With a
>> fs_mark workload that creates 40 million files we get the following results
>> (all
>> in files/sec)
>>
>> Btrfs Patched Unpatched
>> Average Files/sec: 72209.3 63254.2
>> p50 Files/sec: 70850 57560
>> p90 Files/sec: 68757 53085
>> p99 Files/sec: 68757 53085
>>
>> XFS Patched Unpatched
>> Average Files/sec: 61025.5 60719.5
>> p50 Files/sec: 60107 59465
>> p90 Files/sec: 59300 57966
>> p99 Files/sec: 59227 57528
>>
>> Ext4 Patched Unpatched
>> Average Files/sec: 121785.4 119248.0
>> p50 Files/sec: 120852 119274
>> p90 Files/sec: 116703 112783
>> p99 Files/sec: 114393 104934
>>
>> The reason Btrfs has a much larger improvement is because it holds a lot more
>> things in memory so benefits more from faster slab reclaim, but across the
>> board
>> is an improvement for each of the file systems.
>>
>> Signed-off-by: Josef Bacik <jba...@fb.com>
>> ---
>> fs/dcache.c | 15 ++++++++++-----
>> fs/inode.c | 5 ++++-
>> 2 files changed, 14 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/dcache.c b/fs/dcache.c
>> index 5c7cc95..a558075 100644
>> --- a/fs/dcache.c
>> +++ b/fs/dcache.c
>> @@ -779,8 +779,6 @@ repeat:
>> goto kill_it;
>> }
>>
>> - if (!(dentry->d_flags & DCACHE_REFERENCED))
>> - dentry->d_flags |= DCACHE_REFERENCED;
>> dentry_lru_add(dentry);
>>
>> dentry->d_lockref.count--;
>> @@ -803,6 +801,13 @@ static inline void __dget_dlock(struct dentry *dentry)
>> dentry->d_lockref.count++;
>> }
>>
>> +static inline void __dget_dlock_reference(struct dentry *dentry)
>> +{
>> + if (dentry->d_flags & DCACHE_LRU_LIST)
>> + dentry->d_flags |= DCACHE_REFERENCED;
>> + dentry->d_lockref.count++;
>> +}
>> +
>> static inline void __dget(struct dentry *dentry)
>> {
>> lockref_get(&dentry->d_lockref);
>> @@ -875,7 +880,7 @@ again:
>> (alias->d_flags & DCACHE_DISCONNECTED)) {
>> discon_alias = alias;
>> } else {
>> - __dget_dlock(alias);
>> + __dget_dlock_reference(alias);
>> spin_unlock(&alias->d_lock);
>> return alias;
>> }
>> @@ -886,7 +891,7 @@ again:
>> alias = discon_alias;
>> spin_lock(&alias->d_lock);
>> if (S_ISDIR(inode->i_mode) || !d_unhashed(alias)) {
>> - __dget_dlock(alias);
>> + __dget_dlock_reference(alias);
>> spin_unlock(&alias->d_lock);
>> return alias;
>> }
>> @@ -2250,7 +2255,7 @@ struct dentry *__d_lookup(const struct dentry *parent,
>> const struct qstr *name)
>> if (!d_same_name(dentry, parent, name))
>> goto next;
>>
>> - dentry->d_lockref.count++;
>> + __dget_dlock_reference(dentry);
>
> This misses dentries that we get through __d_lookup_rcu(), so I think
> your change made it so most things aren't getting DCACHE_REFERENCED set
> at all. Maybe something like this instead?
>
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 5c7cc95..d7a56a8 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -412,15 +412,6 @@ static void d_lru_shrink_move(struct list_lru_one *lru,
> struct dentry *dentry,
> list_lru_isolate_move(lru, &dentry->d_lru, list);
> }
>
> -/*
> - * dentry_lru_(add|del)_list) must be called with d_lock held.
> - */
> -static void dentry_lru_add(struct dentry *dentry)
> -{
> - if (unlikely(!(dentry->d_flags & DCACHE_LRU_LIST)))
> - d_lru_add(dentry);
> -}
> -
> /**
> * d_drop - drop a dentry
> * @dentry: dentry to drop
> @@ -779,9 +770,12 @@ void dput(struct dentry *dentry)
> goto kill_it;
> }
>
> - if (!(dentry->d_flags & DCACHE_REFERENCED))
> - dentry->d_flags |= DCACHE_REFERENCED;
> - dentry_lru_add(dentry);
> + if (likely(dentry->d_flags & DCACHE_LRU_LIST)) {
> + if (!(dentry->d_flags & DCACHE_REFERENCED))
> + dentry->d_flags |= DCACHE_REFERENCED;
There is no benefit to checking if DCACHE_REFERENCED is unset before
trying to set it. You've already accessed the cacheline, so you can
avoid the branch here and just set it unconditionally.
Cheers, Andreas
> + } else {
> + d_lru_add(dentry);
> + }
>
> dentry->d_lockref.count--;
> spin_unlock(&dentry->d_lock);
> diff --git a/fs/inode.c b/fs/inode.c
> index 88110fd..16faca3 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -798,6 +798,8 @@ static struct inode *find_inode(struct super_block *sb,
> __wait_on_freeing_inode(inode);
> goto repeat;
> }
> + if (!list_empty(&inode->i_lru))
> + inode->i_state |= I_REFERENCED;
> __iget(inode);
> spin_unlock(&inode->i_lock);
> return inode;
> @@ -825,6 +827,8 @@ static struct inode *find_inode_fast(struct super_block
> *sb,
> __wait_on_freeing_inode(inode);
> goto repeat;
> }
> + if (!list_empty(&inode->i_lru))
> + inode->i_state |= I_REFERENCED;
> __iget(inode);
> spin_unlock(&inode->i_lock);
> return inode;
> @@ -1492,7 +1496,6 @@ static void iput_final(struct inode *inode)
> drop = generic_drop_inode(inode);
>
> if (!drop && (sb->s_flags & MS_ACTIVE)) {
> - inode->i_state |= I_REFERENCED;
> inode_add_lru(inode);
> spin_unlock(&inode->i_lock);
> return;
>
> --
> Omar
> --
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Cheers, Andreas
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