Re: [PATCH 5/6] md/raid5: scale the default stripe cache size with system memory

[yu kuai]

Hi,

在 2026/7/10 21:23, Hiroshi Nishida 写道:
> setup_conf() starts every array with min_nr_stripes = NR_STRIPES (256),
> about 12MB of stripe cache on a 12-disk array.  That fixed default was
> chosen for small systems and is never revisited: a server with hundreds
> of GB of RAM backing a wide array still creates its arrays with the same
> 256 stripes, and only benefits from more after an administrator writes
> stripe_cache_size by hand.
>
> Auto-size the initial count from memory when the new
> stripe_cache_size_default module parameter is 0 (the default): keep the
> historical NR_STRIPES up to RAID5_CACHE_DEFAULT_BASE_GB (8GB) of RAM, then
> grow the count using about 1/512 of the RAM above that baseline, capped at
> RAID5_CACHE_DEFAULT_MAX (4096).  A system with 8GB or less is therefore
> unchanged -- same count, same footprint -- while larger ones scale up
> smoothly (no jump at the baseline) to a bounded maximum.  A non-zero
> stripe_cache_size_default sets a fixed initial size for administrators who
> want one; existing arrays are unaffected, and a reshape still forces at
> least its window's worth of stripes.

I will not accept changing the default value for now, unless it's tested and
validated across various architectures and environments.

BTW, if you want to do this, I'll suggest you write a user space tool.

>
> Signed-off-by: Hiroshi Nishida <nishidafmly@xxxxxxxxx>
> ---
>   drivers/md/raid5.c | 44 +++++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 41 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 5f0825c5effe..8e1c2eba4241 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -83,6 +83,11 @@ module_param(max_stripe_batch, uint, 0644);
>   MODULE_PARM_DESC(max_stripe_batch,
>   		 "Number of stripes a worker thread handles per device_lock acquisition, 1-32 (default 8).  Larger values amortise the lock over more stripes on busy multi-threaded arrays at some latency cost.  Read when an array is created");
>   
> +static unsigned int stripe_cache_size_default;
> +module_param(stripe_cache_size_default, uint, 0644);
> +MODULE_PARM_DESC(stripe_cache_size_default,
> +		 "Initial stripe_cache_size for newly created arrays.  0 (the default) auto-sizes it from system memory: the historical 256 on small hosts, scaling up with RAM to a capped maximum on larger ones.  A non-zero value sets a fixed initial size.  Existing arrays are unaffected");
> +
>   static bool devices_handle_discard_safely = false;
>   module_param(devices_handle_discard_safely, bool, 0644);
>   MODULE_PARM_DESC(devices_handle_discard_safely,
> @@ -7568,6 +7573,17 @@ static unsigned long raid5_cache_count(struct shrinker *shrink,
>   	return max_stripes - min_stripes;
>   }
>   
> +/*
> + * Auto-sizing of the initial stripe cache (stripe_cache_size_default == 0):
> + * stay at the historical NR_STRIPES up to RAID5_CACHE_DEFAULT_BASE_GB of RAM,
> + * then grow the count using about 1/512 of the RAM above that base, capped at
> + * RAID5_CACHE_DEFAULT_MAX.  So a system at or below the base is unchanged and
> + * keeps the historical footprint, while larger ones scale.
> + */
> +#define RAID5_CACHE_DEFAULT_BASE_GB	8	/* unchanged at or below this much RAM */
> +#define RAID5_CACHE_DEFAULT_RAM_SHIFT	9	/* above it: ~1/512 of the extra RAM */
> +#define RAID5_CACHE_DEFAULT_MAX		4096
> +
>   static struct r5conf *setup_conf(struct mddev *mddev)
>   {
>   	struct r5conf *conf;
> @@ -7801,15 +7817,37 @@ static struct r5conf *setup_conf(struct mddev *mddev)
>   		conf->prev_algo = conf->algorithm;
>   	}
>   
> -	conf->min_nr_stripes = NR_STRIPES;
> +	/*
> +	 * Choose the initial stripe cache size.  stripe_cache_size_default
> +	 * selects it: 0 (the default) auto-sizes from memory -- the historical
> +	 * NR_STRIPES up to RAID5_CACHE_DEFAULT_BASE_GB of RAM, then scaling up
> +	 * to at most RAID5_CACHE_DEFAULT_MAX -- and a non-zero value sets it
> +	 * directly.  A reshape still forces at least enough stripes for its
> +	 * window, below.
> +	 */
> +	if (stripe_cache_size_default) {
> +		conf->min_nr_stripes = clamp_t(unsigned long,
> +					stripe_cache_size_default, 16, INT_MAX);
> +	} else {
> +		unsigned long per_stripe = sizeof(struct stripe_head) +
> +			max_disks * (sizeof(struct bio) + PAGE_SIZE);
> +		unsigned long ram = totalram_pages() << PAGE_SHIFT;
> +		unsigned long base = (unsigned long)RAID5_CACHE_DEFAULT_BASE_GB << 30;
> +		unsigned long extra = ram > base ?
> +			((ram - base) >> RAID5_CACHE_DEFAULT_RAM_SHIFT) / per_stripe : 0;
> +
> +		conf->min_nr_stripes = clamp_t(unsigned long, NR_STRIPES + extra,
> +					       NR_STRIPES, RAID5_CACHE_DEFAULT_MAX);
> +	}
>   	if (mddev->reshape_position != MaxSector) {
>   		int stripes = max_t(int,
>   			((mddev->chunk_sectors << 9) / RAID5_STRIPE_SIZE(conf)) * 4,
>   			((mddev->new_chunk_sectors << 9) / RAID5_STRIPE_SIZE(conf)) * 4);
> -		conf->min_nr_stripes = max(NR_STRIPES, stripes);
> -		if (conf->min_nr_stripes != NR_STRIPES)
> +		if (stripes > conf->min_nr_stripes) {
> +			conf->min_nr_stripes = stripes;
>   			pr_info("md/raid:%s: force stripe size %d for reshape\n",
>   				mdname(mddev), conf->min_nr_stripes);
> +		}
>   	}
>   	memory = conf->min_nr_stripes * (sizeof(struct stripe_head) +
>   		 max_disks * ((sizeof(struct bio) + PAGE_SIZE))) / 1024;

-- 
Thanks,
Kuai