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;
> >
> > ....
> >
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@xxxxxxxxx. For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
--
Kind regards,
Minchan Kim
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>