Re: [PATCH v3] zram: remove global tb_lock with fine grain lock
From: Minchan Kim
Date: Tue Jun 03 2014 - 04:21:36 EST
Hello,
On Tue, Jun 03, 2014 at 03:59:06PM +0800, Weijie Yang wrote:
> On Mon, Jun 2, 2014 at 8:43 AM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> > Hello Weijie,
> >
> > Thanks for resending.
> > Below are mostly nitpicks.
> >
> > On Fri, May 30, 2014 at 04:34:44PM +0800, Weijie Yang 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
> >>
> >> 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.
> >>
> >> 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.
> >
> > Nice description. :)
> >
> >>
> >> Changes since v1: https://lkml.org/lkml/2014/5/5/1
> >> - replace CAS method with bit_spinlock method
> >> - rename zram_test_flag() to zram_test_zero()
> >> - add some comments
> >>
> >> Changes since v2: https://lkml.org/lkml/2014/5/15/113
> >> - change size type from int to size_t in zram_set_obj_size()
> >> - refactor zram_set_obj_size() to make it readable
> >> - add comments
> >>
> >> Signed-off-by: Weijie Yang <weijie.yang@xxxxxxxxxxx>
> >> ---
> >> drivers/block/zram/zram_drv.c | 89 ++++++++++++++++++++++++-----------------
> >> drivers/block/zram/zram_drv.h | 22 +++++++---
> >> 2 files changed, 68 insertions(+), 43 deletions(-)
> >>
> >> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> >> index 9849b52..166e882 100644
> >> --- a/drivers/block/zram/zram_drv.c
> >> +++ b/drivers/block/zram/zram_drv.c
> >> @@ -179,23 +179,32 @@ static ssize_t comp_algorithm_store(struct device *dev,
> >> return len;
> >> }
> >>
> >> -/* flag operations needs meta->tb_lock */
> >> -static int zram_test_flag(struct zram_meta *meta, u32 index,
> >> - enum zram_pageflags flag)
> >> +static int zram_test_zero(struct zram_meta *meta, u32 index)
> >
> > Why do you want to create specific function for zero?
> > It would be one of usecase for various potential flags.
> > Do you want to create new functions whenever we define new flag?
> > Or something do you have a mind?
> >
>
> As you see, this patch adds a new flag ZRAM_ACCESS, which is
> accessed through different method from ZRAM_ZERO.
> I think it is hard to use a general method to access all kinds of flags,
> to eliminate some potential ambiguity or wrong usage, I use specific
> function to access different flags.
Although I am not against that, we can do it when it's really messy
as another patch in future. I don't want to bloat patch size without
the goal which is to enhance locking contention.
>
> >
> >> {
> >> - return meta->table[index].flags & BIT(flag);
> >> + return meta->table[index].value & BIT(ZRAM_ZERO);
> >> }
> >>
> >> -static void zram_set_flag(struct zram_meta *meta, u32 index,
> >> - enum zram_pageflags flag)
> >> +static void zram_set_zero(struct zram_meta *meta, u32 index)
> >> {
> >> - meta->table[index].flags |= BIT(flag);
> >> + meta->table[index].value |= BIT(ZRAM_ZERO);
> >> }
> >>
> >> -static void zram_clear_flag(struct zram_meta *meta, u32 index,
> >> - enum zram_pageflags flag)
> >> +static void zram_clear_zero(struct zram_meta *meta, u32 index)
> >> {
> >> - meta->table[index].flags &= ~BIT(flag);
> >> + meta->table[index].value &= ~BIT(ZRAM_ZERO);
> >> +}
> >> +
> >> +static size_t zram_get_obj_size(struct zram_meta *meta, u32 index)
> >> +{
> >> + return meta->table[index].value & (BIT(ZRAM_FLAG_SHIFT) - 1);
> >> +}
> >> +
> >> +static void zram_set_obj_size(struct zram_meta *meta,
> >> + u32 index, size_t size)
> >> +{
> >> + unsigned long flags = meta->table[index].value >> ZRAM_FLAG_SHIFT;
> >> +
> >> + meta->table[index].value = (flags << ZRAM_FLAG_SHIFT) | size;
> >> }
> >>
> >> static inline int is_partial_io(struct bio_vec *bvec)
> >> @@ -255,7 +264,6 @@ static struct zram_meta *zram_meta_alloc(u64 disksize)
> >> goto free_table;
> >> }
> >>
> >> - rwlock_init(&meta->tb_lock);
> >> return meta;
> >>
> >> free_table:
> >> @@ -304,19 +312,24 @@ static void handle_zero_page(struct bio_vec *bvec)
> >> flush_dcache_page(page);
> >> }
> >>
> >> -/* NOTE: caller should hold meta->tb_lock with write-side */
> >> +/*
> >> + * To protect concurrent access to the same index entry,
> >> + * caller should hold this table index entry's bit_spinlock to
> >> + * indicate this index entry is accessing.
> >> + */
> >> static void zram_free_page(struct zram *zram, size_t index)
> >> {
> >> struct zram_meta *meta = zram->meta;
> >> unsigned long handle = meta->table[index].handle;
> >> + size_t 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);
> >> }
> >> return;
> >> @@ -324,27 +337,28 @@ static void zram_free_page(struct zram *zram, size_t index)
> >>
> >> zs_free(meta->mem_pool, handle);
> >>
> >> - atomic64_sub(meta->table[index].size, &zram->stats.compr_data_size);
> >> + size = zram_get_obj_size(meta, index);
> >> + atomic64_sub(size, &zram->stats.compr_data_size);
> >> atomic64_dec(&zram->stats.pages_stored);
> >>
> >> meta->table[index].handle = 0;
> >> - meta->table[index].size = 0;
> >> + zram_set_obj_size(meta, index, 0);
> >> }
> >>
> >> static int zram_decompress_page(struct zram *zram, char *mem, u32 index)
> >> {
> >> - int ret = 0;
> >
> > Unnecessary change.
>
> I want to compact the memory usage on stack, so I put ret and size variables
> together. On 64-bit system, it will be helpful.
Please, do it as another patchset if you think it's worthy.
>
> >> unsigned char *cmem;
> >> struct zram_meta *meta = zram->meta;
> >> unsigned long handle;
> >> - u16 size;
> >
> > I'm not sure it's good idea to use size_t instead of u16 because we apparently
> > have a limitation to express range of size due to packing it into unsigned long
> > so u16 is more clear to show the limiation and someone might find a problem
> > more easily in future if we break something subtle.
> >
> >> + size_t size;
> >> + int ret = 0;
> >>
> >> - read_lock(&meta->tb_lock);
> >> + bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> >> handle = meta->table[index].handle;
> >> - size = meta->table[index].size;
> >> + size = zram_get_obj_size(meta, index);
> >>
> >> - if (!handle || zram_test_flag(meta, index, ZRAM_ZERO)) {
> >> - read_unlock(&meta->tb_lock);
> >> + if (!handle || zram_test_zero(meta, index)) {
> >> + bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> >> clear_page(mem);
> >> return 0;
> >> }
> >> @@ -355,7 +369,7 @@ static int zram_decompress_page(struct zram *zram, char *mem, u32 index)
> >> else
> >> ret = zcomp_decompress(zram->comp, cmem, size, mem);
> >> zs_unmap_object(meta->mem_pool, handle);
> >> - read_unlock(&meta->tb_lock);
> >> + bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> >>
> >> /* Should NEVER happen. Return bio error if it does. */
> >> if (unlikely(ret)) {
> >> @@ -376,14 +390,14 @@ static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
> >> struct zram_meta *meta = zram->meta;
> >> page = bvec->bv_page;
> >>
> >> - read_lock(&meta->tb_lock);
> >> + bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> >> if (unlikely(!meta->table[index].handle) ||
> >> - zram_test_flag(meta, index, ZRAM_ZERO)) {
> >> - read_unlock(&meta->tb_lock);
> >> + zram_test_zero(meta, index)) {
> >> + bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> >> handle_zero_page(bvec);
> >> return 0;
> >> }
> >> - read_unlock(&meta->tb_lock);
> >> + bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> >>
> >> if (is_partial_io(bvec))
> >> /* Use a temporary buffer to decompress the page */
> >> @@ -461,10 +475,10 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> >> if (page_zero_filled(uncmem)) {
> >> kunmap_atomic(user_mem);
> >> /* Free memory associated with this sector now. */
> >> - write_lock(&zram->meta->tb_lock);
> >> + bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> >> zram_free_page(zram, index);
> >> - zram_set_flag(meta, index, ZRAM_ZERO);
> >> - write_unlock(&zram->meta->tb_lock);
> >> + zram_set_zero(meta, index);
> >> + bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> >>
> >> atomic64_inc(&zram->stats.zero_pages);
> >> ret = 0;
> >> @@ -514,12 +528,12 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> >> * Free memory associated with this sector
> >> * before overwriting unused sectors.
> >> */
> >> - write_lock(&zram->meta->tb_lock);
> >> + bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> >> zram_free_page(zram, index);
> >>
> >> meta->table[index].handle = handle;
> >> - meta->table[index].size = clen;
> >> - write_unlock(&zram->meta->tb_lock);
> >> + zram_set_obj_size(meta, index, clen);
> >> + bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> >>
> >> /* Update stats */
> >> atomic64_add(clen, &zram->stats.compr_data_size);
> >> @@ -560,6 +574,7 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> >> int offset, struct bio *bio)
> >> {
> >> size_t n = bio->bi_iter.bi_size;
> >> + struct zram_meta *meta = zram->meta;
> >>
> >> /*
> >> * zram manages data in physical block size units. Because logical block
> >> @@ -584,9 +599,9 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> >> * Discard request can be large so the lock hold times could be
> >> * lengthy. So take the lock once per page.
> >> */
> >> - write_lock(&zram->meta->tb_lock);
> >> + bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> >> zram_free_page(zram, index);
> >> - write_unlock(&zram->meta->tb_lock);
> >> + bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> >> index++;
> >> n -= PAGE_SIZE;
> >> }
> >> @@ -804,9 +819,9 @@ static void zram_slot_free_notify(struct block_device *bdev,
> >> zram = bdev->bd_disk->private_data;
> >> meta = zram->meta;
> >>
> >> - write_lock(&meta->tb_lock);
> >> + bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> >> zram_free_page(zram, index);
> >> - write_unlock(&meta->tb_lock);
> >> + bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> >> atomic64_inc(&zram->stats.notify_free);
> >> }
> >>
> >> diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> >> index 7f21c14..71bc4ad 100644
> >> --- a/drivers/block/zram/zram_drv.h
> >> +++ b/drivers/block/zram/zram_drv.h
> >> @@ -51,10 +51,22 @@ static const size_t max_zpage_size = PAGE_SIZE / 4 * 3;
> >> #define ZRAM_SECTOR_PER_LOGICAL_BLOCK \
> >> (1 << (ZRAM_LOGICAL_BLOCK_SHIFT - SECTOR_SHIFT))
> >>
> >> -/* Flags for zram pages (table[page_no].flags) */
> >> +/*
> >> + * The lower ZRAM_FLAG_SHIFT bits of table.value is for
> >> + * object size (excluding header), the higher bits is for
> >> + * zram_pageflags. By this means, it won't increase any
> >> + * memory overhead on both 32-bit and 64-bit system.
> >
> > Comment on "By this means, ~ 64 bit system" is unncessary because
> > someone read this line but don't know history couldn't understand
> > what's the old structure.
>
> I will remove it.
>
> >> + * zram is mostly used on small embedded system, so we
> >> + * don't want to increase memory footprint. That is why
> >> + * we pack size and flag into table.value.
> >> + */
> >
> > IMHO, it would be more clear but not sure if native speakers look at. ;-)
> >
> > * zram is mainly used for memory efficiency so we want to keep memory
> > * footprint small so we can squeeze size and flags into a field.
> > * The lower ZRAM_FLAG_SHIFT bits is for object size (excluding header),
> > * the higher bits is for zram_pageflags.
> >
>
> That is a better comment.
>
> >> +#define ZRAM_FLAG_SHIFT 24
> >
> > Why is it 24? We have used for 16-bit for size.
> > Do you think it's too small for size?
>
> The reason why I choose 24-bit and size_t is that when I checked the PAGE_SHIFT
> on all kinds of architectures, I found on some architectures such as powerpc and
> hexagon, PAGE_SHIFT would be 18 or 20, so I think 16-bit could be small to use.
If it's a problem, could you do it as another patch? Because it's bug fix,
not related to this patchset.
Thanks.
--
Kind regards,
Minchan Kim
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