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. > >> { >> - 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. >> 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. >> + >> +/* Flags for zram pages (table[page_no].value) */ >> enum zram_pageflags { >> /* Page consists entirely of zeros */ >> - ZRAM_ZERO, >> + ZRAM_ZERO = ZRAM_FLAG_SHIFT + 1, >> + ZRAM_ACCESS, /* page in now accessed */ >> >> __NR_ZRAM_PAGEFLAGS, >> }; >> @@ -64,9 +76,8 @@ enum zram_pageflags { >> /* Allocated for each disk page */ >> struct table { >> unsigned long handle; >> - u16 size; /* object size (excluding header) */ >> - u8 flags; >> -} __aligned(4); >> + unsigned long value; >> +}; >> >> struct zram_stats { >> atomic64_t compr_data_size; /* compressed size of pages stored */ >> @@ -81,7 +92,6 @@ struct zram_stats { >> }; >> >> struct zram_meta { >> - rwlock_t tb_lock; /* protect table */ >> struct table *table; >> struct zs_pool *mem_pool; >> }; >> -- >> 1.7.10.4 >> >> >> -- >> 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>