[libcamera-devel,v4,7/7] libcamera: V4L2BufferCache: Improve cache eviction strategy
diff mbox series

Message ID 20200305203808.536050-8-niklas.soderlund@ragnatech.se
State Accepted
Headers show
Series
  • libcamera: V4L2BufferCache: Improve cache eviction strategy
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Commit Message

Niklas Söderlund March 5, 2020, 8:38 p.m. UTC 
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 the cache was not used as good as it
could for imported buffers.

Improve this by adding a last usage sequence number 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@ragnatech.se>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
---
* Changes since v3
- Group C++ header includes.
- s/UINT32_MAX/UINT64_MAX/
- Use std::memory_order_acq_rel instead of std::memory_order_seq_cst
  order to fetch_add() of the atomic sequence number.
* Changes since v2
- Use a sequence number instead of timestamp to judge age of a cached
  entry.
---
 src/libcamera/include/v4l2_videodevice.h |  7 +++++--
 src/libcamera/v4l2_videodevice.cpp       | 23 +++++++++++++++--------
 2 files changed, 20 insertions(+), 10 deletions(-)

Patch
diff mbox series 

diff --git a/src/libcamera/include/v4l2_videodevice.h b/src/libcamera/include/v4l2_videodevice.h
index 359b366454e4e782..04802012d5315d67 100644
--- a/src/libcamera/include/v4l2_videodevice.h
+++ b/src/libcamera/include/v4l2_videodevice.h
@@ -7,11 +7,12 @@ 
 #ifndef __LIBCAMERA_V4L2_VIDEODEVICE_H__
 #define __LIBCAMERA_V4L2_VIDEODEVICE_H__
 
+#include <atomic>
+#include <memory>
 #include <string>
 #include <vector>
 
 #include <linux/videodev2.h>
-#include <memory>
 
 #include <libcamera/buffer.h>
 #include <libcamera/geometry.h>
@@ -120,11 +121,12 @@  private:
 	{
 	public:
 		Entry();
-		Entry(bool free, const FrameBuffer &buffer);
+		Entry(bool free, uint64_t lastUsed, const FrameBuffer &buffer);
 
 		bool operator==(const FrameBuffer &buffer) const;
 
 		bool free;
+		uint64_t lastUsed;
 
 	private:
 		struct Plane {
@@ -140,6 +142,7 @@  private:
 		std::vector<Plane> planes_;
 	};
 
+	std::atomic_uint64_t lastUsedCounter_;
 	std::vector<Entry> cache_;
 	/* \todo Expose the miss counter through an instrumentation API. */
 	unsigned int missCounter_;
diff --git a/src/libcamera/v4l2_videodevice.cpp b/src/libcamera/v4l2_videodevice.cpp
index c495de85f26efe14..ca0d147f5fc1ea58 100644
--- a/src/libcamera/v4l2_videodevice.cpp
+++ b/src/libcamera/v4l2_videodevice.cpp
@@ -162,7 +162,7 @@  LOG_DECLARE_CATEGORY(V4L2)
  * buffer import, with buffers added to the cache as they are queued.
  */
 V4L2BufferCache::V4L2BufferCache(unsigned int numEntries)
-	: missCounter_(0)
+	: lastUsedCounter_(1), missCounter_(0)
 {
 	cache_.resize(numEntries);
 }
@@ -176,10 +176,12 @@  V4L2BufferCache::V4L2BufferCache(unsigned int numEntries)
  * allocated.
  */
 V4L2BufferCache::V4L2BufferCache(const std::vector<std::unique_ptr<FrameBuffer>> &buffers)
-	: missCounter_(0)
+	: lastUsedCounter_(1), missCounter_(0)
 {
 	for (const std::unique_ptr<FrameBuffer> &buffer : buffers)
-		cache_.emplace_back(true, buffer->planes());
+		cache_.emplace_back(true,
+				    lastUsedCounter_.fetch_add(1, std::memory_order_acq_rel),
+				    buffer->planes());
 }
 
 V4L2BufferCache::~V4L2BufferCache()
@@ -205,6 +207,7 @@  int V4L2BufferCache::get(const FrameBuffer &buffer)
 {
 	bool hit = false;
 	int use = -1;
+	uint64_t oldest = UINT64_MAX;
 
 	for (unsigned int index = 0; index < cache_.size(); index++) {
 		const Entry &entry = cache_[index];
@@ -219,8 +222,10 @@  int V4L2BufferCache::get(const FrameBuffer &buffer)
 			break;
 		}
 
-		if (use < 0)
+		if (entry.lastUsed < oldest) {
 			use = index;
+			oldest = entry.lastUsed;
+		}
 	}
 
 	if (!hit)
@@ -229,7 +234,9 @@  int V4L2BufferCache::get(const FrameBuffer &buffer)
 	if (use < 0)
 		return -ENOENT;
 
-	cache_[use] = Entry(false, buffer);
+	cache_[use] = Entry(false,
+			    lastUsedCounter_.fetch_add(1, std::memory_order_acq_rel),
+			    buffer);
 
 	return use;
 }
@@ -245,12 +252,12 @@  void V4L2BufferCache::put(unsigned int index)
 }
 
 V4L2BufferCache::Entry::Entry()
-	: free(true)
+	: free(true), lastUsed(0)
 {
 }
 
-V4L2BufferCache::Entry::Entry(bool free, const FrameBuffer &buffer)
-	: free(free)
+V4L2BufferCache::Entry::Entry(bool free, uint64_t lastUsed, const FrameBuffer &buffer)
+	: free(free), lastUsed(lastUsed)
 {
 	for (const FrameBuffer::Plane &plane : buffer.planes())
 		planes_.emplace_back(plane);