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Date:   Thu, 21 Nov 2019 11:30:38 +0300
From:   Artem Blagodarenko <artem.blagodarenko@...il.com>
To:     Alex Zhuravlev <azhuravlev@...mcloud.com>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [RFC] improve malloc for large filesystems

Hello Alex,

Code looks good, but I have objections about the approach.

512TB disk with 4k block size have 4194304 groups. So 4k groups is only ~0.01% of whole disk.
Can we make decision to break search and get minimum blocks based on such limited data.
I am not sure that spending some time to find good group is worse then allocate blocks without 
optimisation. Especially, if disk is quite free and there are a lot of free block groups.

Best regards,
Artem Blagodarenko.
> On 21 Nov 2019, at 10:03, Alex Zhuravlev <azhuravlev@...mcloud.com> wrote:
> 
> 
> 
>> On 20 Nov 2019, at 21:13, Theodore Y. Ts'o <tytso@....edu> wrote:
>> 
>> Hi Alex,
>> 
>> A couple of comments.  First, please separate this patch so that these
>> two separate pieces of functionality can be reviewed and tested
>> separately:
>> 
> 
> This is the first patch of the series.
> 
> Thanks, Alex
> 
> From 81c4b3b5a17d94525bbc6d2d89b20f6618b05bc6 Mon Sep 17 00:00:00 2001
> From: Alex Zhuravlev <bzzz@...mcloud.com>
> Date: Thu, 21 Nov 2019 09:53:13 +0300
> Subject: [PATCH 1/2] ext4: limit scanning for a good group
> 
> at first two rounds to prevent situation when 10x-100x thousand
> of groups are scanned, especially non-initialized groups.
> 
> Signed-off-by: Alex Zhuravlev <bzzz@...mcloud.com>
> ---
> fs/ext4/ext4.h    |  2 ++
> fs/ext4/mballoc.c | 14 ++++++++++++--
> fs/ext4/sysfs.c   |  4 ++++
> 3 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 03db3e71676c..d4e47fdad87c 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1480,6 +1480,8 @@ struct ext4_sb_info {
> 	/* where last allocation was done - for stream allocation */
> 	unsigned long s_mb_last_group;
> 	unsigned long s_mb_last_start;
> +	unsigned int s_mb_toscan0;
> +	unsigned int s_mb_toscan1;
> 
> 	/* stats for buddy allocator */
> 	atomic_t s_bal_reqs;	/* number of reqs with len > 1 */
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index a3e2767bdf2f..cebd7d8df0b8 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2098,7 +2098,7 @@ static int ext4_mb_good_group(struct ext4_allocation_context *ac,
> static noinline_for_stack int
> ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> {
> -	ext4_group_t ngroups, group, i;
> +	ext4_group_t ngroups, toscan, group, i;
> 	int cr;
> 	int err = 0, first_err = 0;
> 	struct ext4_sb_info *sbi;
> @@ -2169,7 +2169,15 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> 		 */
> 		group = ac->ac_g_ex.fe_group;
> 
> -		for (i = 0; i < ngroups; group++, i++) {
> +		/* limit number of groups to scan at the first two rounds
> +		 * when we hope to find something really good */
> +		toscan = ngroups;
> +		if (cr == 0)
> +			toscan = sbi->s_mb_toscan0;
> +		else if (cr == 1)
> +			toscan = sbi->s_mb_toscan1;
> +
> +		for (i = 0; i < toscan; group++, i++) {
> 			int ret = 0;
> 			cond_resched();
> 			/*
> @@ -2872,6 +2880,8 @@ void ext4_process_freed_data(struct super_block *sb, tid_t commit_tid)
> 			bio_put(discard_bio);
> 		}
> 	}
> +	sbi->s_mb_toscan0 = 1024;
> +	sbi->s_mb_toscan1 = 4096;
> 
> 	list_for_each_entry_safe(entry, tmp, &freed_data_list, efd_list)
> 		ext4_free_data_in_buddy(sb, entry);
> diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
> index eb1efad0e20a..c96ee20f5487 100644
> --- a/fs/ext4/sysfs.c
> +++ b/fs/ext4/sysfs.c
> @@ -198,6 +198,8 @@ EXT4_RO_ATTR_ES_UI(errors_count, s_error_count);
> EXT4_ATTR(first_error_time, 0444, first_error_time);
> EXT4_ATTR(last_error_time, 0444, last_error_time);
> EXT4_ATTR(journal_task, 0444, journal_task);
> +EXT4_RW_ATTR_SBI_UI(mb_toscan0, s_mb_toscan0);
> +EXT4_RW_ATTR_SBI_UI(mb_toscan1, s_mb_toscan1);
> 
> static unsigned int old_bump_val = 128;
> EXT4_ATTR_PTR(max_writeback_mb_bump, 0444, pointer_ui, &old_bump_val);
> @@ -228,6 +230,8 @@ static struct attribute *ext4_attrs[] = {
> 	ATTR_LIST(first_error_time),
> 	ATTR_LIST(last_error_time),
> 	ATTR_LIST(journal_task),
> +	ATTR_LIST(mb_toscan0),
> +	ATTR_LIST(mb_toscan1),
> 	NULL,
> };
> ATTRIBUTE_GROUPS(ext4);
> -- 
> 2.20.1
> 
> 

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