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#ifndef TIMESTAMP_POOL_HH_
#define TIMESTAMP_POOL_HH_
// The purpose of this file is to provide the definition of a 'timestamp pool'.
// It manages blocks of timestamp query pools, hands them out when requested,
// and allocates more when (if) we run out. It also efficiently reads them back.
// This class solves some key issues:
//
// 1. We need a potentially infinite amount of timestamps available to the
// GPU. While I imagine most (good) applications will limit the amount of
// times they call vkQueueSubmit, there's no bound we can place on the
// amount of times this function will be called. Also,
// the amount of frames in flight might vary, so really we need
// num_queue_submits * max_frames_in_flight timestamps. Obviously, we don't
// know what these numbers are at runtime and can't assume that they are
// reasonable or even constant either. We solve this by allocating more
// timestamps when necessary.
// 2. We don't want to hammer vulkan with expensive timestamp read
// operations. If we have hundreds of query pools lying around, reading them
// back will take hundreds of individual vulkan calls. They
// should be batched as to perform as few reads as possible. So if we allocate
// multiple big query pool strips, then reading them will only require that many
// calls. We then can cache off the result of reading as well so iterating
// through later doesn't require any vulkan interaction at all.
//
//
// Usage:
// 1. Get handle with .acquire().
// 2. Write start/end timestamp operations with the handle's pool and index
// into the provided command buffer.
// 3. With the command buffer signalled completion via some semaphore /
// fence, call .poll(). This will cache off all outstanding handles.
// Retrieving with handles which have not been signalled are undefined.
// 4. Retrieve timestamp results with .get_polled(your_handle).
// 5. Destruct the handle to return the key to the pool.
#include <vulkan/utility/vk_dispatch_table.h>
#include <vulkan/vulkan.hpp>
#include <memory>
#include <unordered_set>
#include <vector>
namespace low_latency {
class QueueContext;
class TimestampPool final {
private:
QueueContext& queue_context;
// A chunk of data which is useful for making timestamp queries.
// Allows association of an index to a query pool and command buffer.
// We reuse these when they're released.
struct QueryChunk final {
private:
using free_indices_t = std::unordered_set<std::uint64_t>;
static constexpr auto CHUNK_SIZE = 512u;
public:
VkQueryPool query_pool;
std::unique_ptr<free_indices_t> free_indices;
std::unique_ptr<std::vector<VkCommandBuffer>> command_buffers;
public:
QueryChunk(const QueueContext& queue_context);
QueryChunk(const QueryChunk& handle) = delete;
QueryChunk(QueryChunk&&) = delete;
QueryChunk operator=(const QueryChunk& handle) = delete;
QueryChunk operator=(QueryChunk&&) = delete;
~QueryChunk();
};
std::unordered_set<std::shared_ptr<QueryChunk>> query_chunks;
public:
// A handle represents a VkCommandBuffer and a query index.
// Once the Handle goes out of scope, the query index will be returned
// to the parent pool.
struct Handle final {
private:
friend class TimestampPool;
private:
const std::weak_ptr<QueryChunk> origin_chunk;
public:
const VkQueryPool query_pool;
const std::uint64_t query_index;
const VkCommandBuffer command_buffer;
public:
Handle(const std::shared_ptr<QueryChunk>& origin_chunk,
const std::uint64_t& query_index);
Handle(const Handle& handle) = delete;
Handle(Handle&&) = delete;
Handle operator=(const Handle& handle) = delete;
Handle operator=(Handle&&) = delete;
~Handle();
public:
void setup_command_buffers(const Handle& tail,
const QueueContext& queue_context) const;
std::optional<std::uint64_t> get_ticks(const TimestampPool& pool);
};
public:
TimestampPool(QueueContext& queue_context);
TimestampPool(const TimestampPool&) = delete;
TimestampPool(TimestampPool&&) = delete;
TimestampPool operator=(const TimestampPool&) = delete;
TimestampPool operator=(TimestampPool&&) = delete;
~TimestampPool();
public:
// Hands out a Handle!
std::shared_ptr<Handle> acquire();
};
} // namespace low_latency
#endif
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