Re: [Qemu-devel] [PATCH 3/7] XBZRLE: optimize XBZRLE to decrease the cache missing

[Dr. David Alan Gilbert]

* Gonglei (Arei) (address@hidden) wrote:
> Avoid the hot pages cache replacing by others to remarkable decrease cache
> missing. The counter of updating dirty bitmap is used to indicate the cached
> page age.

It seems worth noting that you've changed 'it_age's use - that's ok, but
perhaps not obvious.

Also, the change in the behaviour of cache_insert to 'fail' when it decides
not to insert a page is not obvious.


> Signed-off-by: ChenLiang <address@hidden>
> Signed-off-by: Gonglei <address@hidden>
> ---
>  arch_init.c                    |  8 +++++---
>  include/migration/page_cache.h |  8 ++++++--
>  page_cache.c                   | 19 +++++++++++++++----
>  3 files changed, 26 insertions(+), 9 deletions(-)
> 
> diff --git a/arch_init.c b/arch_init.c
> index 6823c5a..fc71331 100644
> --- a/arch_init.c
> +++ b/arch_init.c
> @@ -315,7 +315,8 @@ static void xbzrle_cache_zero_page(ram_addr_t 
> current_addr)
>  
>      /* We don't care if this fails to allocate a new cache page
>       * as long as it updated an old one */
> -    cache_insert(XBZRLE.cache, current_addr, ZERO_TARGET_PAGE);
> +    cache_insert(XBZRLE.cache, current_addr, ZERO_TARGET_PAGE,
> +                                              get_bitmap_sync_cnt());
>  }
>  
>  #define ENCODING_FLAG_XBZRLE 0x1
> @@ -327,10 +328,11 @@ static int save_xbzrle_page(QEMUFile *f, uint8_t 
> **current_data,
>      int encoded_len = 0, bytes_sent = -1;
>      uint8_t *prev_cached_page;
>  
> -    if (!cache_is_cached(XBZRLE.cache, current_addr)) {
> +    if (!cache_is_cached(XBZRLE.cache, current_addr, get_bitmap_sync_cnt())) 
> {
>          acct_info.xbzrle_cache_miss++;
>          if (!last_stage) {
> -            if (cache_insert(XBZRLE.cache, current_addr, *current_data) == 
> -1) {
> +            if (cache_insert(XBZRLE.cache, current_addr, *current_data,
> +                                               get_bitmap_sync_cnt()) == -1) 
> {
>                  return -1;
>              } else {
>                  /* update *current_data when the page has been
> diff --git a/include/migration/page_cache.h b/include/migration/page_cache.h
> index 2d5ce2d..cadce55 100644
> --- a/include/migration/page_cache.h
> +++ b/include/migration/page_cache.h
> @@ -43,8 +43,10 @@ void cache_fini(PageCache *cache);
>   *
>   * @cache pointer to the PageCache struct
>   * @addr: page addr
> + * @current_age indicate age of the page if cache hit
>   */
> -bool cache_is_cached(const PageCache *cache, uint64_t addr);
> +bool cache_is_cached(const PageCache *cache, uint64_t addr,
> +                                             uint64_t current_age);
>
>  /**
>   * get_cached_data: Get the data cached for an addr
> @@ -65,8 +67,10 @@ uint8_t *get_cached_data(const PageCache *cache, uint64_t 
> addr);
>   * @cache pointer to the PageCache struct
>   * @addr: page address
>   * @pdata: pointer to the page
> + * @current_age indicate age of the page if the page is inserted into cache
>   */
> -int cache_insert(PageCache *cache, uint64_t addr, const uint8_t *pdata);
> +int cache_insert(PageCache *cache, uint64_t addr, const uint8_t *pdata,
> +                                                  uint64_t current_age);

The comment on cache_insert needs changing, it currently says:
 * Returns -1 on error

that's not true any more, it'll return -1 on error or if it just decided
that it's better not to cache it.

>  /**
>   * cache_resize: resize the page cache. In case of size reduction the extra
> diff --git a/page_cache.c b/page_cache.c
> index b033681..fa58ab2 100644
> --- a/page_cache.c
> +++ b/page_cache.c
> @@ -121,7 +121,8 @@ static size_t cache_get_cache_pos(const PageCache *cache,
>      return pos;
>  }
>  
> -bool cache_is_cached(const PageCache *cache, uint64_t addr)
> +bool cache_is_cached(const PageCache *cache, uint64_t addr,
> +                                             uint64_t current_age)
>  {
>      size_t pos;
>  
> @@ -130,7 +131,12 @@ bool cache_is_cached(const PageCache *cache, uint64_t 
> addr)
>  
>      pos = cache_get_cache_pos(cache, addr);
>  
> -    return (cache->page_cache[pos].it_addr == addr);
> +    if (cache->page_cache[pos].it_addr == addr) {
> +        /* updata the it_age when the cache hit */

Typo: updata -> update

> +        cache->page_cache[pos].it_age = current_age;
> +        return true;
> +    }
> +    return false;
>  }
>  
>  static CacheItem *cache_get_by_addr(const PageCache *cache, uint64_t addr)
> @@ -150,7 +156,8 @@ uint8_t *get_cached_data(const PageCache *cache, uint64_t 
> addr)
>      return cache_get_by_addr(cache, addr)->it_data;
>  }
>  
> -int cache_insert(PageCache *cache, uint64_t addr, const uint8_t *pdata)
> +int cache_insert(PageCache *cache, uint64_t addr, const uint8_t *pdata,
> +                                                        uint64_t current_age)
>  {
>  
>      CacheItem *it = NULL;
> @@ -161,6 +168,10 @@ int cache_insert(PageCache *cache, uint64_t addr, const 
> uint8_t *pdata)
>      /* actual update of entry */
>      it = cache_get_by_addr(cache, addr);
>  
> +    if ((it->it_data != NULL) && (it->it_age + 2 > current_age)) {
> +        /* the cache page is fresh, don't replace it */
> +        return -1;
> +    }

A magical constant '2' - should probably a define/const somewhere.

>      /* allocate page */
>      if (!it->it_data) {
>          it->it_data = g_try_malloc(cache->page_size);
> @@ -173,7 +184,7 @@ int cache_insert(PageCache *cache, uint64_t addr, const 
> uint8_t *pdata)
>  
>      memcpy(it->it_data, pdata, cache->page_size);
>  
> -    it->it_age = ++cache->max_item_age;
> +    it->it_age = current_age;
>      it->it_addr = addr;
>  
>      return 0;

Dave
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK