Re: [PATCH v6 6/6] zswap: shrinks zswap pool based on memory pressure

[Andrew Morton]

On Mon, 27 Nov 2023 11:37:03 -0800 Nhat Pham <nphamcs@xxxxxxxxx> wrote:

> Currently, we only shrink the zswap pool when the user-defined limit is
> hit. This means that if we set the limit too high, cold data that are
> unlikely to be used again will reside in the pool, wasting precious
> memory. It is hard to predict how much zswap space will be needed ahead
> of time, as this depends on the workload (specifically, on factors such
> as memory access patterns and compressibility of the memory pages).
> 
> This patch implements a memcg- and NUMA-aware shrinker for zswap, that
> is initiated when there is memory pressure. The shrinker does not
> have any parameter that must be tuned by the user, and can be opted in
> or out on a per-memcg basis.
> 
> Furthermore, to make it more robust for many workloads and prevent
> overshrinking (i.e evicting warm pages that might be refaulted into
> memory), we build in the following heuristics:
> 
> * Estimate the number of warm pages residing in zswap, and attempt to
>   protect this region of the zswap LRU.
> * Scale the number of freeable objects by an estimate of the memory
>   saving factor. The better zswap compresses the data, the fewer pages
>   we will evict to swap (as we will otherwise incur IO for relatively
>   small memory saving).
> * During reclaim, if the shrinker encounters a page that is also being
>   brought into memory, the shrinker will cautiously terminate its
>   shrinking action, as this is a sign that it is touching the warmer
>   region of the zswap LRU.
> 
> As a proof of concept, we ran the following synthetic benchmark:
> build the linux kernel in a memory-limited cgroup, and allocate some
> cold data in tmpfs to see if the shrinker could write them out and
> improved the overall performance. Depending on the amount of cold data
> generated, we observe from 14% to 35% reduction in kernel CPU time used
> in the kernel builds.
> 
> ...
>
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -22,6 +22,7 @@
>  #include <linux/mm_types.h>
>  #include <linux/page-flags.h>
>  #include <linux/local_lock.h>
> +#include <linux/zswap.h>
>  #include <asm/page.h>
>  
>  /* Free memory management - zoned buddy allocator.  */
> @@ -641,6 +642,7 @@ struct lruvec {
>  #ifdef CONFIG_MEMCG
>  	struct pglist_data *pgdat;
>  #endif
> +	struct zswap_lruvec_state zswap_lruvec_state;

Normally we'd put this in #ifdef CONFIG_ZSWAP.

> --- a/include/linux/zswap.h
> +++ b/include/linux/zswap.h
> @@ -5,20 +5,40 @@
>  #include <linux/types.h>
>  #include <linux/mm_types.h>
>  
> +struct lruvec;
> +
>  extern u64 zswap_pool_total_size;
>  extern atomic_t zswap_stored_pages;
>  
>  #ifdef CONFIG_ZSWAP
>  
> +struct zswap_lruvec_state {
> +	/*
> +	 * Number of pages in zswap that should be protected from the shrinker.
> +	 * This number is an estimate of the following counts:
> +	 *
> +	 * a) Recent page faults.
> +	 * b) Recent insertion to the zswap LRU. This includes new zswap stores,
> +	 *    as well as recent zswap LRU rotations.
> +	 *
> +	 * These pages are likely to be warm, and might incur IO if the are written
> +	 * to swap.
> +	 */
> +	atomic_long_t nr_zswap_protected;
> +};
> +
>  bool zswap_store(struct folio *folio);
>  bool zswap_load(struct folio *folio);
>  void zswap_invalidate(int type, pgoff_t offset);
>  void zswap_swapon(int type);
>  void zswap_swapoff(int type);
>  void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg);
> -
> +void zswap_lruvec_state_init(struct lruvec *lruvec);
> +void zswap_lruvec_swapin(struct page *page);
>  #else
>  
> +struct zswap_lruvec_state {};

But instead you made it an empty struct in this case.

That's a bit funky, but I guess OK.  It does send a careful reader of
struct lruvec over to look at the zswap_lruvec_state definition to
understand what's going on.

>  static inline bool zswap_store(struct folio *folio)
>  {
>  	return false;
> @@ -33,7 +53,8 @@ static inline void zswap_invalidate(int type, pgoff_t offset) {}
>  static inline void zswap_swapon(int type) {}
>  static inline void zswap_swapoff(int type) {}
>  static inline void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg) {}
> -
> +static inline void zswap_lruvec_init(struct lruvec *lruvec) {}
> +static inline void zswap_lruvec_swapin(struct page *page) {}

Needed this build fix:

--- a/include/linux/zswap.h~zswap-shrinks-zswap-pool-based-on-memory-pressure-fix
+++ a/include/linux/zswap.h
@@ -54,6 +54,7 @@ static inline void zswap_swapon(int type
 static inline void zswap_swapoff(int type) {}
 static inline void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg) {}
 static inline void zswap_lruvec_init(struct lruvec *lruvec) {}
+static inline void zswap_lruvec_state_init(struct lruvec *lruvec) {}
 static inline void zswap_lruvec_swapin(struct page *page) {}
 #endif
 
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