Date:	Sat, 17 May 2014 09:34:31 +0800
From:	Weijie Yang <weijie.yang.kh@...il.com>
To:	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Weijie Yang <weijie.yang@...sung.com>,
	Nitin Gupta <ngupta@...are.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Bob Liu <bob.liu@...cle.com>,
	Dan Streetman <ddstreet@...e.org>,
	Heesub Shin <heesub.shin@...sung.com>,
	Davidlohr Bueso <davidlohr@...com>,
	Joonsoo Kim <js1304@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v2] zram: remove global tb_lock with fine grain lock

On Fri, May 16, 2014 at 2:51 PM, Minchan Kim <minchan@...nel.org> wrote:
> 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@...sung.com> 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?
>>

Yes, I did. However, I think it is hard the use seqlocks here, no
matter use it as
a meta global lock or a table[index] lock. The main reason is the
writer will free
the handle rather than just change some values.

>> > 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.
>

This data is just for reference: for 1GB zram, CAS will increase about
1MB memory
on 32-bit system, the other locks will increase more especially when we config
DEBUG_SPINLOCK these configs.
Consider the zsmalloc compress ratio (about 1: 4.7 on test), we can save more
memory, it is good news for embedded system.

>>
>> > 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.
>

I agree with Minchan, zram is a general block device, we should consider
the completeness of its logic.

But your question really inspire me, maybe we can modify the frontswap/zswap
system by removing any inner lock, because it is for special purpose and the
upper swap system already have its lock logic.

>>
>> >
>> > ....
>> >
>> >  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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