Messages in this thread | ![/](/images/icornerl.gif) | | Date | Mon, 17 Mar 2003 15:11:08 +0000 | From | Matthew Wilcox <> | Subject | Re: [Ext2-devel] [PATCH] distributed counters for ext2 to avoid group scaning |
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On Mon, Mar 17, 2003 at 12:55:17AM +0300, Alex Tomas wrote: > > Also, while your goal is to reduce cache ping-pong between CPUs, we will > > now have cache ping-pong for the "diff" array. We need to do per-cpu > > values or make each value cacheline aligned to avoid ping-pong. > > yes, you're right. fixed
Um. But ... let's say I'm on a P4 box running a distro kernel. I suspect most distros will set NR_CPUs to 8. So that's 128 byte cachelines * 8 CPUs gives 1024 bytes -- 1k. Per dcounter (never mind the size of seqlock or dc_base/dc_min):
> +struct dcounter_diff { > + long dd_value; > +} ____cacheline_aligned_in_smp; > + > +struct dcounter { > + long dc_base; > + long dc_min; > + struct dcounter_diff dc_diff[NR_CPUS]; > + seqlock_t dc_lock; > +};
> +++ edited/include/linux/ext2_fs_sb.h Mon Mar 17 00:12:00 2003 > struct ext2_bg_info *s_bgi; > + struct dcounter free_blocks_dc; > + struct dcounter free_inodes_dc; > + struct dcounter dirs_dc;
And then we have 3 of these (an additional 3k..). Per blockgroup. My 4GB / has 30 blockgroups; my 30GB /home has 232. So that's a little under 8 per GB. My _laptop_ has a 40GB drive, so that's on the order of 320 blockgroups -- almost an additional megabyte of ram consumed for these counters.
Maybe it makes big boxes go faster, but this makes my dual CPU desktop go slower, and that's not acceptable.
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