Re: [PATCH v2] zram: remove global tb_lock with fine grain lock

[Minchan Kim]

Hello Andrew,

On Thu, May 15, 2014 at 02:38:56PM -0700, Andrew Morton wrote:
> On Thu, 15 May 2014 16:00:47 +0800 Weijie Yang <weijie.yang@xxxxxxxxxxx> wrote:
> 
> > Currently, we use a rwlock tb_lock to protect concurrent access to
> > the whole zram meta table. However, according to the actual access model,
> > there is only a small chance for upper user to access the same table[index],
> > so the current lock granularity is too big.
> > 
> > The idea of optimization is to change the lock granularity from whole
> > meta table to per table entry (table -> table[index]), so that we can
> > protect concurrent access to the same table[index], meanwhile allow
> > the maximum concurrency.
> > With this in mind, several kinds of locks which could be used as a
> > per-entry lock were tested and compared:
> > 
> > Test environment:
> > x86-64 Intel Core2 Q8400, system memory 4GB, Ubuntu 12.04,
> > kernel v3.15.0-rc3 as base, zram with 4 max_comp_streams LZO.
> > 
> > iozone test:
> > iozone -t 4 -R -r 16K -s 200M -I +Z
> > (1GB zram with ext4 filesystem, take the average of 10 tests, KB/s)
> > 
> >       Test       base      CAS    spinlock    rwlock   bit_spinlock
> > -------------------------------------------------------------------
> >  Initial write  1381094   1425435   1422860   1423075   1421521
> >        Rewrite  1529479   1641199   1668762   1672855   1654910
> >           Read  8468009  11324979  11305569  11117273  10997202
> >        Re-read  8467476  11260914  11248059  11145336  10906486
> >   Reverse Read  6821393   8106334   8282174   8279195   8109186
> >    Stride read  7191093   8994306   9153982   8961224   9004434
> >    Random read  7156353   8957932   9167098   8980465   8940476
> > Mixed workload  4172747   5680814   5927825   5489578   5972253
> >   Random write  1483044   1605588   1594329   1600453   1596010
> >         Pwrite  1276644   1303108   1311612   1314228   1300960
> >          Pread  4324337   4632869   4618386   4457870   4500166
> 
> Did you investigate seqlocks?
> 
> > To enhance the possibility of access the same table[index] concurrently,
> > set zram a small disksize(10MB) and let threads run with large loop count.
> > 
> > fio test:
> > fio --bs=32k --randrepeat=1 --randseed=100 --refill_buffers
> > --scramble_buffers=1 --direct=1 --loops=3000 --numjobs=4
> > --filename=/dev/zram0 --name=seq-write --rw=write --stonewall
> > --name=seq-read --rw=read --stonewall --name=seq-readwrite
> > --rw=rw --stonewall --name=rand-readwrite --rw=randrw --stonewall
> > (10MB zram raw block device, take the average of 10 tests, KB/s)
> > 
> >     Test     base     CAS    spinlock    rwlock  bit_spinlock
> > -------------------------------------------------------------
> > seq-write   933789   999357   1003298    995961   1001958
> >  seq-read  5634130  6577930   6380861   6243912   6230006
> >    seq-rw  1405687  1638117   1640256   1633903   1634459
> >   rand-rw  1386119  1614664   1617211   1609267   1612471
> > 
> > All the optimization methods show a higher performance than the base,
> > however, it is hard to say which method is the most appropriate.
> > 
> > On the other hand, zram is mostly used on small embedded system, so we
> > don't want to increase any memory footprint.
> > 
> > This patch pick the bit_spinlock method, pack object size and page_flag
> > into an unsigned long table.value, so as to not increase any memory
> > overhead on both 32-bit and 64-bit system.
> 
> bit_spinlocks are not a particularly good or complete mechanism - they
> don't have lockdep support and iirc they are somewhat slow.
> 
> So we need a pretty good reason to use them.  How much memory saving
> are we expecting here?

Actually, the reason would be same with page->flags bit spinlock.
Given that normally people set up swap size two times bigger than
memory, zram table's bloating will be bigger than struct page's one.

> 
> > On the third hand, even though different kinds of locks have different
> > performances, we can ignore this difference, because:
> > if zram is used as zram swapfile, the swap subsystem can prevent concurrent
> > access to the same swapslot;
> > if zram is used as zram-blk for set up filesystem on it, the upper filesystem
> > and the page cache also prevent concurrent access of the same block mostly.
> > So we can ignore the different performances among locks.
> 
> So do we need any locking at all?

Yes, insane user might want to read/write block device directly while
another user uses it with some FS on the block device so at least, zram
should make sure consistency.

> 
> >
> > ....
> >
> >  static void zram_free_page(struct zram *zram, size_t index)
> >  {
> >  	struct zram_meta *meta = zram->meta;
> >  	unsigned long handle = meta->table[index].handle;
> > +	int size;
> >  
> >  	if (unlikely(!handle)) {
> >  		/*
> >  		 * No memory is allocated for zero filled pages.
> >  		 * Simply clear zero page flag.
> >  		 */
> > -		if (zram_test_flag(meta, index, ZRAM_ZERO)) {
> > -			zram_clear_flag(meta, index, ZRAM_ZERO);
> > +		if (zram_test_zero(meta, index)) {
> > +			zram_clear_zero(meta, index);
> >  			atomic64_dec(&zram->stats.zero_pages);
> 
> Having these atomic ops in the alloc/free hotpaths must be costing us?

Yeb, maybe but I think it's not a scope of this patch. If it was really
trouble, maybe we could change accouting with percpu.

Thanks.

> 
> >  		}
> >  		return;
> >
> > ....
> >
> 
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-- 
Kind regards,
Minchan Kim
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