On Fri, May 16, 2014 at 2:51 PM, Minchan Kim <minchan@xxxxxxxxxx> 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@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?
>>
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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