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Subject: Re: [PATCH] ext4: Optimize ext4 DIO overwrites
To:     Jan Kara <jack@suse.cz>, Ted Tso <tytso@mit.edu>
Cc:     linux-ext4@vger.kernel.org,
        Dan Williams <dan.j.williams@intel.com>,
        "Berrocal, Eduardo" <eduardo.berrocal@intel.com>
References: <20191218174433.19380-1-jack@suse.cz>
From:   Ritesh Harjani <riteshh@linux.ibm.com>
Date:   Thu, 19 Dec 2019 19:23:28 +0530
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On 12/18/19 11:14 PM, Jan Kara wrote:
> Currently we start transaction for mapping every extent for writing
> using direct IO. This is unnecessary when we know we are overwriting
> already allocated blocks and the overhead of starting a transaction can
> be significant especially for multithreaded workloads doing small writes.
> Use iomap operations that avoid starting a transaction for direct IO
> overwrites.
> 
> This improves throughput of 4k random writes - fio jobfile:
> [global]
> rw=randrw
> norandommap=1
> invalidate=0
> bs=4k
> numjobs=16
> time_based=1
> ramp_time=30
> runtime=120
> group_reporting=1
> ioengine=psync
> direct=1
> size=16G
> filename=file1.0.0:file1.0.1:file1.0.2:file1.0.3:file1.0.4:file1.0.5:file1.0.6:file1.0.7:file1.0.8:file1.0.9:file1.0.10:file1.0.11:file1.0.12:file1.0.13:file1.0.14:file1.0.15:file1.0.16:file1.0.17:file1.0.18:file1.0.19:file1.0.20:file1.0.21:file1.0.22:file1.0.23:file1.0.24:file1.0.25:file1.0.26:file1.0.27:file1.0.28:file1.0.29:file1.0.30:file1.0.31
> file_service_type=random
> nrfiles=32
> 
> from 3018MB/s to 4059MB/s in my test VM running test against simulated
> pmem device (note that before iomap conversion, this workload was able
> to achieve 3708MB/s because old direct IO path avoided transaction start
> for overwrites as well). For dax, the win is even larger improving
> throughput from 3042MB/s to 4311MB/s.

However for dax via ext4_dax_write_iter() path, we still need a way to
detect if it's overwrite and that path can be optimized too right?
I see, that this path could use both `shared inode locking` and
`no journal transaction` optimizations in case of overwrites. Correct?


> 
> Reported-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Jan Kara <jack@suse.cz>

This was one of the next AI I too wanted to do. I guess since everyone
loves performance improvements. :)

No problem with current patch. Looks good. Gave it a run too on my
system.

Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>


However depending on which patch lands first one may need a
re-basing. Will conflict with this-
https://marc.info/?l=linux-ext4&m=157613016931238&w=2


> ---
>   fs/ext4/ext4.h  |  1 +
>   fs/ext4/file.c  |  4 +++-
>   fs/ext4/inode.c | 21 +++++++++++++++++++++
>   3 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index f8578caba40d..e31fc5749a19 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -3390,6 +3390,7 @@ static inline void ext4_clear_io_unwritten_flag(ext4_io_end_t *io_end)
>   }
> 
>   extern const struct iomap_ops ext4_iomap_ops;
> +extern const struct iomap_ops ext4_iomap_overwrite_ops;
>   extern const struct iomap_ops ext4_iomap_report_ops;
> 
>   static inline int ext4_buffer_uptodate(struct buffer_head *bh)
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index 6a7293a5cda2..f8e4af72d64d 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -370,6 +370,7 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
>   	loff_t offset;
>   	handle_t *handle;
>   	struct inode *inode = file_inode(iocb->ki_filp);
> +	const struct iomap_ops *iomap_ops = &ext4_iomap_ops;
>   	bool extend = false, overwrite = false, unaligned_aio = false;
> 
>   	if (iocb->ki_flags & IOCB_NOWAIT) {
> @@ -415,6 +416,7 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
>   	if (!unaligned_aio && ext4_overwrite_io(inode, offset, count) &&
>   	    ext4_should_dioread_nolock(inode)) {
>   		overwrite = true;
> +		iomap_ops = &ext4_iomap_overwrite_ops;
>   		downgrade_write(&inode->i_rwsem);
>   	}
> 
> @@ -435,7 +437,7 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
>   		ext4_journal_stop(handle);
>   	}
> 
> -	ret = iomap_dio_rw(iocb, from, &ext4_iomap_ops, &ext4_dio_write_ops,
> +	ret = iomap_dio_rw(iocb, from, iomap_ops, &ext4_dio_write_ops,
>   			   is_sync_kiocb(iocb) || unaligned_aio || extend);
> 
>   	if (extend)
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 28f28de0c1b6..e1eb4493aacc 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3448,6 +3448,22 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>   	return 0;
>   }
> 
> +static int ext4_iomap_overwrite_begin(struct inode *inode, loff_t offset,
> +		loff_t length, unsigned flags, struct iomap *iomap,
> +		struct iomap *srcmap)
> +{
> +	int ret;
> +
> +	/*
> +	 * Even for writes we don't need to allocate blocks, so just pretend
> +	 * we are reading to save overhead of starting a transaction.
> +	 */
> +	flags &= ~IOMAP_WRITE;
> +	ret = ext4_iomap_begin(inode, offset, length, flags, iomap, srcmap);
> +	WARN_ON_ONCE(iomap->type != IOMAP_MAPPED);
> +	return ret;
> +}
> +
>   static int ext4_iomap_end(struct inode *inode, loff_t offset, loff_t length,
>   			  ssize_t written, unsigned flags, struct iomap *iomap)
>   {
> @@ -3469,6 +3485,11 @@ const struct iomap_ops ext4_iomap_ops = {
>   	.iomap_end		= ext4_iomap_end,
>   };
> 
> +const struct iomap_ops ext4_iomap_overwrite_ops = {
> +	.iomap_begin		= ext4_iomap_overwrite_begin,
> +	.iomap_end		= ext4_iomap_end,
> +};
> +
>   static bool ext4_iomap_is_delalloc(struct inode *inode,
>   				   struct ext4_map_blocks *map)
>   {
>