Happy new year!
Seth, just checking if you have had a chance yet to think about this one.
On Thu, Dec 19, 2013 at 8:23 AM, Dan Streetman <ddstreet@xxxxxxxx> wrote:
> Currently, zswap is writeback cache; stored pages are not sent
> to swap disk, and when zswap wants to evict old pages it must
> first write them back to swap cache/disk manually. This avoids
> swap out disk I/O up front, but only moves that disk I/O to
> the writeback case (for pages that are evicted), and adds the
> overhead of having to uncompress the evicted pages and the
> need for an additional free page (to store the uncompressed page).
>
> This optionally changes zswap to writethrough cache by enabling
> frontswap_writethrough() before registering, so that any
> successful page store will also be written to swap disk. The
> default remains writeback. To enable writethrough, the param
> zswap.writethrough=1 must be used at boot.
>
> Whether writeback or writethrough will provide better performance
> depends on many factors including disk I/O speed/throughput,
> CPU speed(s), system load, etc. In most cases it is likely
> that writeback has better performance than writethrough before
> zswap is full, but after zswap fills up writethrough has
> better performance than writeback.
>
> Signed-off-by: Dan Streetman <ddstreet@xxxxxxxx>
>
> ---
>
> Based on specjbb testing on my laptop, the results for both writeback
> and writethrough are better than not using zswap at all, but writeback
> does seem to be better than writethrough while zswap isn't full. Once
> it fills up, performance for writethrough is essentially close to not
> using zswap, while writeback seems to be worse than not using zswap.
> However, I think more testing on a wider span of systems and conditions
> is needed. Additionally, I'm not sure that specjbb is measuring true
> performance under fully loaded cpu conditions, so additional cpu load
> might need to be added or specjbb parameters modified (I took the
> values from the 4 "warehouses" test run).
>
> In any case though, I think having writethrough as an option is still
> useful. More changes could be made, such as changing from writeback
> to writethrough based on the zswap % full. And the patch doesn't
> change default behavior - writethrough must be specifically enabled.
>
> The %-ized numbers I got from specjbb on average, using the default
> 20% max_pool_percent and varying the amount of heap used as shown:
>
> ram | no zswap | writeback | writethrough
> 75 93.08 100 96.90
> 87 96.58 95.58 96.72
> 100 92.29 89.73 86.75
> 112 63.80 38.66 19.66
> 125 4.79 29.90 15.75
> 137 4.99 4.50 4.75
> 150 4.28 4.62 5.01
> 162 5.20 2.94 4.66
> 175 5.71 2.11 4.84
>
>
>
> mm/zswap.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++----
> 1 file changed, 64 insertions(+), 4 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index e55bab9..2f919db 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -61,6 +61,8 @@ static atomic_t zswap_stored_pages = ATOMIC_INIT(0);
> static u64 zswap_pool_limit_hit;
> /* Pages written back when pool limit was reached */
> static u64 zswap_written_back_pages;
> +/* Pages evicted when pool limit was reached */
> +static u64 zswap_evicted_pages;
> /* Store failed due to a reclaim failure after pool limit was reached */
> static u64 zswap_reject_reclaim_fail;
> /* Compressed page was too big for the allocator to (optimally) store */
> @@ -89,6 +91,10 @@ static unsigned int zswap_max_pool_percent = 20;
> module_param_named(max_pool_percent,
> zswap_max_pool_percent, uint, 0644);
>
> +/* Writeback/writethrough mode (fixed at boot for now) */
> +static bool zswap_writethrough;
> +module_param_named(writethrough, zswap_writethrough, bool, 0444);
> +
> /*********************************
> * compression functions
> **********************************/
> @@ -629,6 +635,48 @@ end:
> }
>
> /*********************************
> +* evict code
> +**********************************/
> +
> +/*
> + * This evicts pages that have already been written through to swap.
> + */
> +static int zswap_evict_entry(struct zbud_pool *pool, unsigned long handle)
> +{
> + struct zswap_header *zhdr;
> + swp_entry_t swpentry;
> + struct zswap_tree *tree;
> + pgoff_t offset;
> + struct zswap_entry *entry;
> +
> + /* extract swpentry from data */
> + zhdr = zbud_map(pool, handle);
> + swpentry = zhdr->swpentry; /* here */
> + zbud_unmap(pool, handle);
> + tree = zswap_trees[swp_type(swpentry)];
> + offset = swp_offset(swpentry);
> + BUG_ON(pool != tree->pool);
> +
> + /* find and ref zswap entry */
> + spin_lock(&tree->lock);
> + entry = zswap_rb_search(&tree->rbroot, offset);
> + if (!entry) {
> + /* entry was invalidated */
> + spin_unlock(&tree->lock);
> + return 0;
> + }
> +
> + zswap_evicted_pages++;
> +
> + zswap_rb_erase(&tree->rbroot, entry);
> + zswap_entry_put(tree, entry);
> +
> + spin_unlock(&tree->lock);
> +
> + return 0;
> +}
> +
> +/*********************************
> * frontswap hooks
> **********************************/
> /* attempts to compress and store an single page */
> @@ -744,7 +792,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
> spin_lock(&tree->lock);
> entry = zswap_entry_find_get(&tree->rbroot, offset);
> if (!entry) {
> - /* entry was written back */
> + /* entry was written back or evicted */
> spin_unlock(&tree->lock);
> return -1;
> }
> @@ -778,7 +826,7 @@ static void zswap_frontswap_invalidate_page(unsigned type, pgoff_t offset)
> spin_lock(&tree->lock);
> entry = zswap_rb_search(&tree->rbroot, offset);
> if (!entry) {
> - /* entry was written back */
> + /* entry was written back or evicted */
> spin_unlock(&tree->lock);
> return;
> }
> @@ -813,18 +861,26 @@ static void zswap_frontswap_invalidate_area(unsigned type)
> zswap_trees[type] = NULL;
> }
>
> -static struct zbud_ops zswap_zbud_ops = {
> +static struct zbud_ops zswap_zbud_writeback_ops = {
> .evict = zswap_writeback_entry
> };
> +static struct zbud_ops zswap_zbud_writethrough_ops = {
> + .evict = zswap_evict_entry
> +};
>
> static void zswap_frontswap_init(unsigned type)
> {
> struct zswap_tree *tree;
> + struct zbud_ops *ops;
>
> tree = kzalloc(sizeof(struct zswap_tree), GFP_KERNEL);
> if (!tree)
> goto err;
> - tree->pool = zbud_create_pool(GFP_KERNEL, &zswap_zbud_ops);
> + if (zswap_writethrough)
> + ops = &zswap_zbud_writethrough_ops;
> + else
> + ops = &zswap_zbud_writeback_ops;
> + tree->pool = zbud_create_pool(GFP_KERNEL, ops);
> if (!tree->pool)
> goto freetree;
> tree->rbroot = RB_ROOT;
> @@ -875,6 +931,8 @@ static int __init zswap_debugfs_init(void)
> zswap_debugfs_root, &zswap_reject_compress_poor);
> debugfs_create_u64("written_back_pages", S_IRUGO,
> zswap_debugfs_root, &zswap_written_back_pages);
> + debugfs_create_u64("evicted_pages", S_IRUGO,
> + zswap_debugfs_root, &zswap_evicted_pages);
> debugfs_create_u64("duplicate_entry", S_IRUGO,
> zswap_debugfs_root, &zswap_duplicate_entry);
> debugfs_create_u64("pool_pages", S_IRUGO,
> @@ -919,6 +977,8 @@ static int __init init_zswap(void)
> pr_err("per-cpu initialization failed\n");
> goto pcpufail;
> }
> + if (zswap_writethrough)
> + frontswap_writethrough(true);
> frontswap_register_ops(&zswap_frontswap_ops);
> if (zswap_debugfs_init())
> pr_warn("debugfs initialization failed\n");
> --
> 1.8.3.1
>
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