Re: [PATCH x11spice v3 3/3] Add cache for SHM segments

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> 
> Add a cache to allow the reuse of SHM segments.
> Shared memory segments are added to the cache instead of being
> deallocated, and the cache is searched instead of/before allocating a
> new segment.
> 
> Both the SHM segments and their attachment with the X server are cached.
> 
> The cache currently has a fixed number of 10 entries, this provided a
> good cache hit rate while keeping memory usage under control.
> Building with DEBUG_SHM_CACHE defined and running with
> G_MESSAGES_DEBUG=all will periodically print out the SHM cache hit
> rate.
> 
> On my Ubuntu 18.04 system running XFCE4 with a 2560x1440 screen, the
> cache hit rate starts around 72%. On-screen windows that update often
> and have consistently-sized damage rectangles are the best case. With
> several of those (scrolling terminal windows, web browser showing a
> WebGL demo), the hit rate slowly rises to around 92%.
> Operations that generate rapid damage reports (like resizing or moving
> windows) will lower the hit rate.
> 
> Signed-off-by: Brendan Shanks <bshanks@xxxxxxxxxxxxxxx>
> ---
>  src/display.c | 175 ++++++++++++++++++++++++++++++++++++++++++++++----
>  src/display.h |  11 +++-
>  2 files changed, 170 insertions(+), 16 deletions(-)
> 
> diff --git a/src/display.c b/src/display.c
> index a2a18b8..55461a2 100644
> --- a/src/display.c
> +++ b/src/display.c
> @@ -213,11 +213,155 @@ static int register_for_events(display_t *d)
>      return 0;
>  }
>  
> +static void shm_segment_destroy(display_t *d, shm_segment_t *segment)
> +{
> +    if (segment->shmid == -1) {
> +        return;
> +    }
> +
> +    xcb_shm_detach(d->c, segment->shmseg);
> +    segment->shmseg = -1;
> +
> +    shmdt(segment->shmaddr);
> +    segment->shmaddr = NULL;
> +
> +    shmctl(segment->shmid, IPC_RMID, NULL);
> +    segment->shmid = -1;
> +}
> +
> +
> +static int shm_cache_get(display_t *d, size_t size, shm_segment_t *segment)
> +{
> +    int i, ret;
> +    shm_segment_t *bigger_entry = NULL;
> +    shm_segment_t *entry_to_use = NULL;
> +
> +#if defined(DEBUG_SHM_CACHE)
> +    static guint cache_hits = 0;
> +    static guint cache_total = 0;
> +
> +    cache_total++;
> +#endif
> +
> +    g_mutex_lock(&d->shm_cache_mutex);
> +
> +    /* Check the cache for a segment of size 'size' or bigger.
> +     * Use an exact-size segment if found.
> +     * If not, use the smallest entry that is big enough.
> +     */
> +    for (i = 0; i < G_N_ELEMENTS(d->shm_cache); i++) {
> +        shm_segment_t *entry = &d->shm_cache[i];
> +
> +        if (entry->shmid != -1) {
> +            /* If a cache entry of the exact size being requested is found,
> use that */
> +            if (size == entry->size) {
> +                entry_to_use = entry;
> +                break;
> +            }
> +
> +            /* Keep track of the next-biggest entry in case an exact-size
> match isn't found */
> +            if (size < entry->size) {
> +                if ((bigger_entry && entry->size < bigger_entry->size) ||
> !bigger_entry) {

This is equivalent to 

   if (!bigger_entry || entry->size < bigger_entry->size) {

> +                    bigger_entry = entry;
> +                }
> +            }
> +        }
> +    }
> +
> +    /* An exact-size entry wasn't found, use the next biggest entry that was
> found */
> +    if (!entry_to_use) {
> +        entry_to_use = bigger_entry;
> +    }
> +
> +    if (entry_to_use) {
> +        *segment = *entry_to_use;
> +        entry_to_use->shmid = -1;
> +
> +        ret = 1;
> +
> +#if defined(DEBUG_SHM_CACHE)
> +        cache_hits++;
> +        if ((cache_hits % 100 == 0)) {
> +            g_debug("SHM cache hitrate: %u/%u (%.2f%%)", cache_hits,
> cache_total,
> +                    (float) ((float) cache_hits / (float) cache_total) *
> 100);
> +        }
> +#endif
> +    } else {
> +        /* No usable entry found in the cache */
> +        ret = 0;
> +    }
> +
> +    g_mutex_unlock(&d->shm_cache_mutex);
> +    return ret;
> +}
> +
> +static int shm_cache_add(display_t *d, shm_segment_t *segment)
> +{
> +    int i, ret;
> +    shm_segment_t *smallest_entry = NULL;
> +    shm_segment_t *entry_to_use = NULL;
> +
> +    g_mutex_lock(&d->shm_cache_mutex);
> +
> +    /* 'segment' is now unused, try to add it to the cache.
> +     * Use an empty slot in the cache if available.
> +     * If not, evict the smallest entry (which also must be smaller than
> 'segment') from the cache.
> +     */
> +    for (i = 0; i < G_N_ELEMENTS(d->shm_cache); i++) {
> +        shm_segment_t *entry = &d->shm_cache[i];
> +
> +        if (entry->shmid == -1) {
> +            /* Use an empty slot if found */
> +            entry_to_use = entry;
> +            break;
> +        }
> +
> +        /* Keep track of the smallest entry that's smaller than 'segment' */
> +        if (entry->size < segment->size) {
> +            if (smallest_entry && entry->size < smallest_entry->size) {
> +                smallest_entry = entry;
> +            } else if (!smallest_entry) {

Similar here (single if),

  if (!smallest_entry || entry->size < smallest_entry->size) {

> +                smallest_entry = entry;
> +            }
> +        }
> +    }
> +
> +    /* If no empty entries were found, evict 'smallest_entry' and reuse it
> */
> +    if (!entry_to_use && smallest_entry) {
> +        shm_segment_destroy(d, smallest_entry);
> +        entry_to_use = smallest_entry;
> +    }
> +
> +    if (entry_to_use) {
> +        *entry_to_use = *segment;
> +        ret = 1;
> +    } else {
> +        /* Cache is full, and contained no entries smaller than the one
> being added */
> +        ret = 0;
> +    }
> +
> +    g_mutex_unlock(&d->shm_cache_mutex);
> +    return ret;
> +}
> +
> +static void shm_cache_destroy(display_t *d)
> +{
> +    int i;
> +
> +    g_mutex_lock(&d->shm_cache_mutex);
> +    for (i = 0; i < G_N_ELEMENTS(d->shm_cache); i++) {
> +        shm_segment_t *entry = &d->shm_cache[i];
> +
> +        shm_segment_destroy(d, entry);
> +    }
> +    g_mutex_unlock(&d->shm_cache_mutex);
> +}
>  
>  int display_open(display_t *d, session_t *session)
>  {
>      int scr;
>      int rc;
> +    int i;
>      xcb_damage_query_version_cookie_t dcookie;
>      xcb_damage_query_version_reply_t *damage_version;
>      xcb_xkb_use_extension_cookie_t use_cookie;
> @@ -314,6 +458,11 @@ int display_open(display_t *d, session_t *session)
>      if (rc)
>          return rc;
>  
> +    g_mutex_init(&d->shm_cache_mutex);
> +    for (i = 0; i < G_N_ELEMENTS(d->shm_cache); i++) {
> +        d->shm_cache[i].shmid = -1;
> +    }
> +
>      rc = display_create_screen_images(d);
>  
>      g_message("Display %s opened", session->options.display ?
>      session->options.display : "");
> @@ -321,17 +470,6 @@ int display_open(display_t *d, session_t *session)
>      return rc;
>  }
>  
> -/*
> -  TODO: Implement a cache for shared memory handles
> -        That is, instead of doing the shmget/shmat for every read,
> -        we should cache the shmid, and reuse if we're doing a later
> -        read of a similar size.
> -
> -        We would likely see a 40% improvement in raw read time.  Note
> -        that a callgrind run suggested that would be on the order of 5%
> -        overall.
> -*/
> -
>  shm_image_t *create_shm_image(display_t *d, unsigned int w, unsigned int h)
>  {
>      shm_image_t *shmi;
> @@ -349,6 +487,11 @@ shm_image_t *create_shm_image(display_t *d, unsigned int
> w, unsigned int h)
>      shmi->bytes_per_line = (bits_per_pixel(d) / 8) * shmi->w;
>      imgsize = shmi->bytes_per_line * shmi->h;
>  
> +    if (shm_cache_get(d, imgsize, &shmi->segment)) {
> +        return shmi;
> +    }
> +
> +    /* No usable shared memory segment found in cache, allocate a new one */
>      shmi->segment.shmid = shmget(IPC_PRIVATE, imgsize, IPC_CREAT | 0700);
>      if (shmi->segment.shmid != -1)
>          shmi->segment.shmaddr = shmat(shmi->segment.shmid, 0, 0);
> @@ -360,6 +503,7 @@ shm_image_t *create_shm_image(display_t *d, unsigned int
> w, unsigned int h)
>      /* We tell shmctl to detach now; that prevents us from holding this
>         shared memory segment forever in case of abnormal process exit. */
>      shmctl(shmi->segment.shmid, IPC_RMID, NULL);
> +    shmi->segment.size = imgsize;
>  
>      shmi->segment.shmseg = xcb_generate_id(d->c);
>      cookie = xcb_shm_attach_checked(d->c, shmi->segment.shmseg,
>      shmi->segment.shmid, 0);
> @@ -451,9 +595,10 @@ void display_copy_image_into_fullscreen(display_t *d,
> shm_image_t *shmi, int x,
>  
>  void destroy_shm_image(display_t *d, shm_image_t *shmi)
>  {
> -    xcb_shm_detach(d->c, shmi->segment.shmseg);
> -    shmdt(shmi->segment.shmaddr);
> -    shmctl(shmi->segment.shmid, IPC_RMID, NULL);
> +    if (!shm_cache_add(d, &shmi->segment)) {
> +        /* Could not add to cache, destroy this segment */
> +        shm_segment_destroy(d, &shmi->segment);
> +    }
>      if (shmi->drawable_ptr)
>          free(shmi->drawable_ptr);
>      free(shmi);
> @@ -521,6 +666,8 @@ void display_stop_event_thread(display_t *d)
>  
>  void display_close(display_t *d)
>  {
> +    shm_cache_destroy(d);
> +    g_mutex_clear(&d->shm_cache_mutex);
>      xcb_damage_destroy(d->c, d->damage);
>      display_destroy_screen_images(d);
>      xcb_disconnect(d->c);
> diff --git a/src/display.h b/src/display.h
> index da1b991..0aea348 100644
> --- a/src/display.h
> +++ b/src/display.h
> @@ -21,18 +21,19 @@
>  #ifndef DISPLAY_H_
>  #define DISPLAY_H_
>  
> +#include <glib.h>
>  #include <xcb/xcb.h>
>  #include <xcb/damage.h>
>  #include <xcb/shm.h>
>  
> -
>  struct session_struct;
>  
>  /*----------------------------------------------------------------------------
>  **  Structure definitions
>  **--------------------------------------------------------------------------*/
>  typedef struct {
> -    int shmid;
> +    int shmid;      /* if shmid is -1: the shm_segment_t is "empty", other
> members are undefined */
> +    size_t size;
>      xcb_shm_seg_t shmseg;
>      void *shmaddr;
>  } shm_segment_t;
> @@ -63,6 +64,12 @@ typedef struct {
>      shm_image_t *fullscreen;
>      shm_image_t *scanline;
>  
> +    /* The SHM cache holds up to 10 segments, this provides a good cache
> +     * hit rate while keeping memory usage reasonable.
> +     */
> +    shm_segment_t shm_cache[10];
> +    GMutex shm_cache_mutex;
> +
>      pthread_t event_thread;
>      struct session_struct *session;
>  } display_t;

It seems fine, last word to Jeremy

Frediano

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