[libcamera-devel] [PATCH 1/2] libcamera: V4L2BufferCache: Improve cache eviction strategy

[Naushir Patuck]

Hi Niklas,


On Sun, 16 Feb 2020 at 16:17, Niklas Söderlund
<niklas.soderlund at ragnatech.se> wrote:
>
> The strategy used to find a free cache entry in the first implementation
> was not the smartest, it picked the first free entry. This lead to
> unwanted performance issues as they cache was not used as good as it
> could for imported buffers.
>
> Improve this by adding a last usage timestamp to the cache entries and
> change the eviction strategy to use the oldest free entry instead of the
> first one it finds.
>
> Signed-off-by: Niklas Söderlund <niklas.soderlund at ragnatech.se>
> ---
>  src/libcamera/include/v4l2_videodevice.h | 1 +
>  src/libcamera/v4l2_videodevice.cpp       | 9 ++++++---
>  2 files changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/src/libcamera/include/v4l2_videodevice.h b/src/libcamera/include/v4l2_videodevice.h
> index fcf072641420dacf..f04feed87b24f28f 100644
> --- a/src/libcamera/include/v4l2_videodevice.h
> +++ b/src/libcamera/include/v4l2_videodevice.h
> @@ -125,6 +125,7 @@ private:
>                 bool operator==(const FrameBuffer &buffer);
>
>                 bool free;
> +               utils::time_point lastUsed;
>
>         private:
>                 struct Plane {
> diff --git a/src/libcamera/v4l2_videodevice.cpp b/src/libcamera/v4l2_videodevice.cpp
> index 99470ce11421c77c..9a4d2479b20f5e27 100644
> --- a/src/libcamera/v4l2_videodevice.cpp
> +++ b/src/libcamera/v4l2_videodevice.cpp
> @@ -205,6 +205,7 @@ int V4L2BufferCache::get(const FrameBuffer &buffer)
>  {
>         bool hit = false;
>         int use = -1;
> +       utils::time_point oldest = utils::clock::now();
>
>         for (unsigned int index = 0; index < cache_.size(); index++) {
>                 const Entry &entry = cache_[index];
> @@ -212,8 +213,10 @@ int V4L2BufferCache::get(const FrameBuffer &buffer)
>                 if (!entry.free)
>                         continue;
>
> -               if (use < 0)
> +               if (cache_[index].lastUsed < oldest) {
>                         use = index;
> +                       oldest = cache_[index].lastUsed;
> +               }
>
>                 /* Try to find a cache hit by comparing the planes. */
>                 if (cache_[index] == buffer) {
> @@ -245,12 +248,12 @@ void V4L2BufferCache::put(unsigned int index)
>  }
>
>  V4L2BufferCache::Entry::Entry()
> -       : free(true)
> +       : free(true), lastUsed(utils::clock::now())
>  {
>  }
>
>  V4L2BufferCache::Entry::Entry(bool free, const FrameBuffer &buffer)
> -       : free(free)
> +       : free(free), lastUsed(utils::clock::now())
>  {
>         for (const FrameBuffer::Plane &plane : buffer.planes())
>                 planes_.emplace_back(plane);

Thank you, this does fix my issue with cache entry under-utilisation.

Reviewed-by: Naushir Patuck <naush at raspberrypi.com>

> --
> 2.25.0
>
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Thanks,
Naush