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Date:   Fri, 10 Nov 2017 15:39:30 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:     linux-ext4 <linux-ext4@...r.kernel.org>,
        Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH] ext4: improve smp scalability for inode generation

On Nov 8, 2017, at 8:23 PM, Theodore Ts'o <tytso@....edu> wrote:
> commit f0e922e7235e1b5ba6fd964e2cf8dafed3248a15
> Author: Theodore Ts'o <tytso@....edu>
> Date:   Wed Nov 8 22:21:58 2017 -0500
> 
>    ext4: improve smp scalability for inode generation
> 
>    ->s_next_generation is protected by s_next_gen_lock but its usage
>    pattern is very primitive.  We don't actually need sequentailly
>    increasing new generation numbers, so let's use prandom_u32() instead.

We discussed this on the ext4 concall this week, but came to the conclusion
that a 32-bit random number has an unacceptably high chance of collision
(about 1/2^16 allocations) due to the birthday paradox.

Instead, an approach like the following should be OK:
1. num_gen_per_cpu = 2^32/num_online_cpus()
2. initialize CPU0 generation = prandom_u32()
3. initialize CPUn generation = CPU0 + n * num_gen_per_cpu
4. use sequential generation numbers for each local CPU
5. after num_gen_per_cpu generations per CPU hit on any CPU, goto 2.

Cheers, Andreas
> 
>    Reported-by: Dmitry Monakhov <dmonakhov@...nvz.org>
>    Signed-off-by: Theodore Ts'o <tytso@....edu>
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 53ce95b52fd8..5e6d7b6f50c7 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1355,8 +1355,6 @@ struct ext4_sb_info {
> 	int s_first_ino;
> 	unsigned int s_inode_readahead_blks;
> 	unsigned int s_inode_goal;
> -	spinlock_t s_next_gen_lock;
> -	u32 s_next_generation;
> 	u32 s_hash_seed[4];
> 	int s_def_hash_version;
> 	int s_hash_unsigned;	/* 3 if hash should be signed, 0 if not */
> diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
> index ee823022aa34..da79eb5dba40 100644
> --- a/fs/ext4/ialloc.c
> +++ b/fs/ext4/ialloc.c
> @@ -1138,9 +1138,7 @@ struct inode *__ext4_new_inode(handle_t *handle, struct inode *dir,
> 			   inode->i_ino);
> 		goto out;
> 	}
> -	spin_lock(&sbi->s_next_gen_lock);
> -	inode->i_generation = sbi->s_next_generation++;
> -	spin_unlock(&sbi->s_next_gen_lock);
> +	inode->i_generation = prandom_u32();
> 
> 	/* Precompute checksum seed for inode metadata */
> 	if (ext4_has_metadata_csum(sb)) {
> diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
> index 144bbda2b808..98ad8172dfd3 100644
> --- a/fs/ext4/ioctl.c
> +++ b/fs/ext4/ioctl.c
> @@ -17,6 +17,7 @@
> #include <linux/uuid.h>
> #include <linux/uaccess.h>
> #include <linux/delay.h>
> +#include <linux/random.h>
> #include "ext4_jbd2.h"
> #include "ext4.h"
> #include <linux/fsmap.h>
> @@ -157,10 +158,8 @@ static long swap_inode_boot_loader(struct super_block *sb,
> 
> 	inode->i_ctime = inode_bl->i_ctime = current_time(inode);
> 
> -	spin_lock(&sbi->s_next_gen_lock);
> -	inode->i_generation = sbi->s_next_generation++;
> -	inode_bl->i_generation = sbi->s_next_generation++;
> -	spin_unlock(&sbi->s_next_gen_lock);
> +	inode->i_generation = prandom_u32();
> +	inode_bl->i_generation = prandom_u32();
> 
> 	ext4_discard_preallocations(inode);
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 3a278faf5868..9f2e3eb5131f 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -3982,8 +3982,6 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
> 	}
> 
> 	sbi->s_gdb_count = db_count;
> -	get_random_bytes(&sbi->s_next_generation, sizeof(u32));
> -	spin_lock_init(&sbi->s_next_gen_lock);
> 
> 	timer_setup(&sbi->s_err_report, print_daily_error_info, 0);
> 


Cheers, Andreas






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