On Thu, 4 Jul 2013, Jan Kara wrote:
> Date: Thu, 4 Jul 2013 17:09:05 +0200
> From: Jan Kara <jack@xxxxxxx>
> To: Lukas Czerner <lczerner@xxxxxxxxxx>
> Cc: linux-ext4@xxxxxxxxxxxxxxx, enwlinux@xxxxxxxxx,
> Jose_Mario_Gallegos@xxxxxxxx, jordan_hargrave@xxxxxxxx,
> rwheeler@xxxxxxxxxx
> Subject: Re: [RFC PATCH 1/1] ext4: Try to better reuse recently freed space
>
> On Thu 04-07-13 11:11:54, Lukas Czerner wrote:
> > Currently if the block allocator can not find the goal to allocate we
> > would use global goal for stream allocation. However the global goal
> > (s_mb_last_group and s_mb_last_start) will move further every time such
> > allocation appears and never move backwards.
> >
> > This causes several problems in certain scenarios:
> >
> > - the goal will move further and further preventing us from reusing
> > space which might have been freed since then. This is ok from the file
> > system point of view because we will reuse that space eventually,
> > however we're allocating block from slower parts of the spinning disk
> > even though it might not be necessary.
> > - The above also causes more serious problem for example for thinly
> > provisioned storage (sparse images backed storage as well), because
> > instead of reusing blocks which are already provisioned we would try
> > to use new blocks. This would unnecessarily drain storage free blocks
> > pool.
> > - This will also cause blocks to be allocated further from the given
> > goal than it's necessary. Consider for example truncating, or removing
> > and rewriting the file in the loop. This workload will never reuse
> > freed blocks until we continually claim and free all the block in the
> > file system.
> >
> > Note that file systems like xfs, ext3, or btrfs does not have this
> > problem. This is simply caused by the notion of global pool.
> >
> > Fix this by changing the global goal to be goal per inode. This will
> > allow us to invalidate the goal every time the inode has been truncated,
> > or newly created, so in those cases we would try to use the proper more
> > specific goal which is based on inode position.
> In principle the patch looks fine to me. Just minor style nit below:
>
> >
> > Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx>
> > ---
> > fs/ext4/ext4.h | 7 ++++---
> > fs/ext4/inode.c | 8 ++++++++
> > fs/ext4/mballoc.c | 20 ++++++++------------
> > 3 files changed, 20 insertions(+), 15 deletions(-)
> >
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index 6ed348d..4dffa92 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -917,6 +917,10 @@ struct ext4_inode_info {
> >
> > /* Precomputed uuid+inum+igen checksum for seeding inode checksums */
> > __u32 i_csum_seed;
> > +
> > + /* where last allocation was done - for stream allocation */
> > + unsigned long i_last_group;
> > + unsigned long i_last_start;
> Can we use proper types here please? I think they should be
> ext4_grpblk_t and ext4_group_t. And maybe we should define something like
> EXT4_INVAL_GRPNO and use that instead of UINT_MAX.
Sure, it'll look better with named types and defined invalidate
value. I'll resend the patch soon.
Thanks!
-Lukas
>
> Honza
> > };
> >
> > /*
> > @@ -1242,9 +1246,6 @@ struct ext4_sb_info {
> > unsigned int s_mb_order2_reqs;
> > unsigned int s_mb_group_prealloc;
> > unsigned int s_max_dir_size_kb;
> > - /* where last allocation was done - for stream allocation */
> > - unsigned long s_mb_last_group;
> > - unsigned long s_mb_last_start;
> >
> > /* stats for buddy allocator */
> > atomic_t s_bal_reqs; /* number of reqs with len > 1 */
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 0188e65..07d0434 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -3702,6 +3702,10 @@ void ext4_truncate(struct inode *inode)
> > else
> > ext4_ind_truncate(handle, inode);
> >
> > + /* Invalidate last allocation counters */
> > + ei->i_last_group = UINT_MAX;
> > + ei->i_last_start = UINT_MAX;
> > +
> And
> > up_write(&ei->i_data_sem);
> >
> > if (IS_SYNC(inode))
> > @@ -4060,6 +4064,10 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
> > inode->i_generation = le32_to_cpu(raw_inode->i_generation);
> > ei->i_block_group = iloc.block_group;
> > ei->i_last_alloc_group = ~0;
> > +
> > + /* Invalidate last allocation counters */
> > + ei->i_last_group = UINT_MAX;
> > + ei->i_last_start = UINT_MAX;
> > /*
> > * NOTE! The in-memory inode i_data array is in little-endian order
> > * even on big-endian machines: we do NOT byteswap the block numbers!
> > diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> > index a9ff5e5..6c23666 100644
> > --- a/fs/ext4/mballoc.c
> > +++ b/fs/ext4/mballoc.c
> > @@ -1591,7 +1591,6 @@ static int mb_mark_used(struct ext4_buddy *e4b, struct ext4_free_extent *ex)
> > static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
> > struct ext4_buddy *e4b)
> > {
> > - struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
> > int ret;
> >
> > BUG_ON(ac->ac_b_ex.fe_group != e4b->bd_group);
> > @@ -1622,10 +1621,8 @@ static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
> > get_page(ac->ac_buddy_page);
> > /* store last allocated for subsequent stream allocation */
> > if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
> > - spin_lock(&sbi->s_md_lock);
> > - sbi->s_mb_last_group = ac->ac_f_ex.fe_group;
> > - sbi->s_mb_last_start = ac->ac_f_ex.fe_start;
> > - spin_unlock(&sbi->s_md_lock);
> > + EXT4_I(ac->ac_inode)->i_last_group = ac->ac_f_ex.fe_group;
> > + EXT4_I(ac->ac_inode)->i_last_start = ac->ac_f_ex.fe_start;
> > }
> > }
> >
> > @@ -2080,13 +2077,12 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> > ac->ac_2order = i - 1;
> > }
> >
> > - /* if stream allocation is enabled, use global goal */
> > - if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
> > - /* TBD: may be hot point */
> > - spin_lock(&sbi->s_md_lock);
> > - ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
> > - ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
> > - spin_unlock(&sbi->s_md_lock);
> > + /* if stream allocation is enabled and per inode goal is
> > + * set, use it */
> > + if ((ac->ac_flags & EXT4_MB_STREAM_ALLOC) &&
> > + (EXT4_I(ac->ac_inode)->i_last_start != UINT_MAX)) {
> > + ac->ac_g_ex.fe_group = EXT4_I(ac->ac_inode)->i_last_group;
> > + ac->ac_g_ex.fe_start = EXT4_I(ac->ac_inode)->i_last_start;
> > }
> >
> > /* Let's just scan groups to find more-less suitable blocks */
> > --
> > 1.8.3.1
> >
> > --
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