[libcamera-devel] [PATCH v2 4/4] libcamera: V4L2BufferCache: Improve cache eviction strategy

[Niklas Söderlund]

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>
Reviewed-by: Naushir Patuck <naush at raspberrypi.com>
---
 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);
-- 
2.25.0