Re: 2.6.35 performance results

[Steven Pratt <slpratt@austin.ibm.com>]

Chris Mason wrote:
> On Mon, Aug 16, 2010 at 04:51:12PM -0500, Steven Pratt wrote:
>   
>> Chris Mason wrote:
>>     
>>>> This changeset introduced "btrfs_start_one_delalloc_inode" in
>>>> http://git.kernel.org/?p=linux/kernel/git/mason/btrfs-unstable.git;a=commitdiff;h=5da9d01b66458b180a6bee0e637a1d0a3effc622
>>>>
>>>> In heavy write workloads this new function is now dominating the profiles:
>>>>
>>>> samples  %        app name                 symbol name
>>>> 8914973  65.1261  btrfs.ko                 btrfs_start_one_delalloc_inode
>>>>         
>>> Hi Steve,
>>>
>>> I think I know why this is a problem and how to fix it, but I'm having a
>>> trouble reproducing this exact setup.  Which of your tests was this
>>> oprofile from?
>>>       
>> 128 thread random write.  With or without nocow option.
>>     
>
> Ok, I haven't managed to reproduce your problem exactly, but this is
> faster for me here.  Could you please give it a try:
>   
Was out on vacation.  Test is running now. Should have results by 
uploaded by Monday.

Steve

> >From 8e965331de749c39f3781d581b55d2c207de060f Mon Sep 17 00:00:00 2001
> From: Chris Mason <chris.mason@oracle.com>
> Date: Wed, 18 Aug 2010 13:31:27 -0400
> Subject: [PATCH] Btrfs: don't trigger delayed allocation throttling as often
>
> We reserve metadata space based on the number of delayed allocation
> extents that are currently pending.  As we run out of space, we start
> forcing writeback to turn those reservations into physical extents.
>
> The reservations are based on some worst case math, so the sooner we
> turn them into real blocks, the better off we are.
>
> But, the writeback is being forced too soon and too often.  This fixes
> things to be less aggressive.
>
> Signed-off-by: Chris Mason <chris.mason@oracle.com>
> ---
>  fs/btrfs/extent-tree.c |    7 ++++++-
>  1 files changed, 6 insertions(+), 1 deletions(-)
>
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 32d0940..55e1ee0 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -3681,6 +3681,7 @@ int btrfs_delalloc_reserve_metadata(struct inode *inode, u64 num_bytes)
>  	struct btrfs_root *root = BTRFS_I(inode)->root;
>  	struct btrfs_block_rsv *block_rsv = &root->fs_info->delalloc_block_rsv;
>  	u64 to_reserve;
> +	u64 max_reserve;
>  	int nr_extents;
>  	int retries = 0;
>  	int ret;
> @@ -3717,7 +3718,11 @@ again:
>  
>  	block_rsv_add_bytes(block_rsv, to_reserve, 1);
>  
> -	if (block_rsv->size > 512 * 1024 * 1024)
> +	/* 10% or 2GB */
> +	max_reserve = min_t(u64, 2ULL * 1024 * 1024 * 1024,
> +		    div_factor(root->fs_info->fs_devices->total_rw_bytes, 1));
> +
> +	if (block_rsv->size > max_reserve)
>  		shrink_delalloc(NULL, root, to_reserve);
>  
>  	return 0;
>