Re: [RFC, PATCH] Avoid hot statistics cache line in ext4 extent cache

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 2012-03-23, at 4:17 PM, Andi Kleen wrote:
> We found that the super block cache line containing the extent cache
> hit/miss statistics counters is very hot in IO workloads.  
> 
> Besides the counter seems racy anyways and can lose updates because
> it's not atomic.
> 
> Disabling the counter make a very measurable difference in some IO
> micro benchmarks on a 4S system.
> 
> I just disabled it for now because it doesn't seem very useful due
> to its racy nature. An alternative would be to turn it into a per
> cpu counter, if someone really needs it.
> 
> Simple patch to disable it appended.
> 
> -Andi

The patch only disables the statistics counting, but not the sysfs
files that will now report bogus statistics.  I don't disagree with
disabling these stats, but if you are disabling the accounting you
should also comment out the functions in fs/ext4/super.c that are
printing out these statistics.

Cheers, Andreas

> ---
> 
> Disable statistics counter in ext4
> 
> This super bloc cache line is very hot and slows down high IO 
> workloads.
> 
> Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 74f23c2..7310b0e 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -2049,10 +2049,12 @@ static int ext4_ext_check_cache(struct inode *inode, ext4_lblk_t block,
> 		ret = 1;
> 	}
> errout:
> +#if 0
> 	if (!ret)
> 		sbi->extent_cache_misses++;
> 	else
> 		sbi->extent_cache_hits++;
> +#endif
> 	trace_ext4_ext_in_cache(inode, block, ret);
> 	spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
> 	return ret;
> 
> -- 
> ak@xxxxxxxxxxxxxxx -- Speaking for myself only
> --
> To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at  http://vger.kernel.org/majordomo-info.html




--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html


[Index of Archives]     [Reiser Filesystem Development]     [Ceph FS]     [Kernel Newbies]     [Security]     [Netfilter]     [Bugtraq]     [Linux FS]     [Yosemite National Park]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Samba]     [Device Mapper]     [Linux Media]

  Powered by Linux