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:
>
> ...
>
> --- 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)
> {
> - 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 int 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, int size)
> +{
> + meta->table[index].value = (unsigned long)size |
> + ((meta->table[index].value >> ZRAM_FLAG_SHIFT)
> + << ZRAM_FLAG_SHIFT );
> }
Let's sort out the types here? It makes no sense for `size' to be
signed. And I don't think we need *any* 64-bit quantities here
(discussed below).
So I think we can make `size' a u32 and remove that typecast.
Also, please use checkpatch ;)
> 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 +312,19 @@ static void handle_zero_page(struct bio_vec *bvec)
> flush_dcache_page(page);
> }
>
> -/* NOTE: caller should hold meta->tb_lock with write-side */
Can we please update this important comment rather than simply deleting
it?
> 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);
> }
> return;
>
> ...
>
> @@ -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;
> +};
Does `value' need to be 64 bit on 64-bit machines? I think u32 will be
sufficient? The struct will still be 16 bytes but if we then play
around adding __packed to this structure we should be able to shrink it
to 12 bytes, save large amounts of memory?
And does `handle' need to be 64-bit on 64-bit?
Problem is, if we make optimisations such as this we will smash head-on
into the bit_spin_lock() requirement that it operate on a ulong*.
Which is due to the bitops requiring a ulong*. How irritating.
um, something like
union table { /* Should be called table_entry */
unsigned long ul;
struct {
u32 size_and_flags;
u32 handle;
} s;
};
That's a 64-bit structure containing 32-bit handle and 8-bit flags and
24-bit size.
I'm tempted to use bitfields here but that could get messy as we handle
endianness.
static void zram_table_lock(union table *table)
{
#ifdef __LITTLE_ENDIAN
bit_spin_lock(ZRAM_ACCESS, &t->ul);
#else
#ifdef CONFIG_64BIT
bit_spin_lock(ZRAM_ACCESS ^ (3 << 3), &t->ul);
#else
bit_spin_lock(ZRAM_ACCESS ^ (7 << 3), &t->ul);
#endif
#endif
}
Or something like that ;) And I don't know if it's correct to use
32-bit handle on 64-bit.
But you get the idea. It's worth spending time over this because the
space savings will be quite large.
> struct zram_stats {
> atomic64_t compr_data_size; /* compressed size of pages stored */
>
> ...
>
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