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115 lines
4.2 KiB
Text
115 lines
4.2 KiB
Text
BufferPools
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-----------
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This document proposes a mechanism to build pools of reusable buffers. The
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proposal should improve performance and help to implement zero-copy usecases.
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Last edited: 2009-09-01 Stefan Kost
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Current Behaviour
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-----------------
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Elements either create own buffers or request downstream buffers via pad_alloc.
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There is hardly any reuse of buffers, instead they are ususaly disposed after
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being rendered.
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Problems
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--------
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- hardware based elements like to reuse buffers as they e.g.
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- mlock them (dsp)
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- establish a index<->address relation (v4l2)
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- not reusing buffers has overhead and makes run time behaviour
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non-deterministic:
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- malloc (which usually becomes an mmap for bigger buffers and thus a
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syscall) and free (can trigger compression of freelists in the allocator)
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- shm alloc/attach, detach/free (xvideo)
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- some usecases cause memcpys
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- not having the right amount of buffers (e.g. too few buffers in v4l2src)
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- receiving buffers of wrong type (e.g. plain buffers in xvimagesink)
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- receiving buffers with wrong alignment (dsp)
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- some usecases cause unneeded cacheflushes when buffers are passed between
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user and kernel-space
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What is needed
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--------------
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Elements that sink raw data buffers of usually constant size would like to
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maintain a bufferpool. These could be sinks or encoders. We need mechanims to
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select and dynamically update:
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- the bufferpool owners in a pipeline
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- the bufferpool sizes
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- the queued buffer sizes, alignments and flags
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Proposal
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--------
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Querying the bufferpool size and buffer alignments can work similar to latency
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queries (gst/gstbin.c:{gst_bin_query,bin_query_latency_fold}. Aggregation is
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quite straight forward : number-of-buffers is summed up and for alignment we
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gather the MAX value.
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Bins need to track which elements have been selected as bufferpools owners and
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update if those are removed (FIXME: in which states?).
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Bins would also need to track if elements that replied to the query are removed
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and update the bufferpool configuration (event). Likewise addition of new
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elements needs to be handled (query and if configuration is changed, update with
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event).
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Bufferpools owners need to handle caps changes to keep the queued buffers valid
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for the negotiated format.
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The bufferpool could be a helper GObject (like we use GstAdapter). If would
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manage a collection of GstBuffers. For each buffer t tracks whether its in use or
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available. The bufferpool in gst-plugin-good/sys/v4l2/gstv4l2bufferpool might be
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a starting point.
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Scenarios
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---------
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v4l2src ! xvimagesink
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~~~~~~~~~~~~~~~~~~~~~
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- v4l2src would report 1 buffer (do we still want the queue-size property?)
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- xvimagesink would report 1 buffer
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v4l2src ! tee name=t ! queue ! xvimagesink t. ! queue ! enc ! mux ! filesink
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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- v4l2src would report 1 buffer
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- xvimagesink would report 1 buffer
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- enc would report 1 buffer
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filesrc ! demux ! queue ! dec ! xvimagesink
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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- dec would report 1 buffer
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- xvimagesink would report 1 buffer
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Issues
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------
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Does it make sense to also have pools for sources or should they always use
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buffers from a downstream element.
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Do we need to add +1 to aggregated buffercount to alloc to have a buffer
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floating? E.g. Can we push buffers queickly enough to have e.g. v4l2src !
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xvimagesink working with 2 buffers. What about v4l2src ! queue ! xvimagesink?
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There are more attributes on buffers needed to reduce the overhead even more:
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- padding: when using buffers on hardware one might need to pad the buffer on
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the end to a specific alignment
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- mlock: hardware that uses DMA needs buffers memory locked, if a buffer is
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already memory locked, it can be used by other hardware based elements as is
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- cache flushes: hardware based elements usually need to flush cpu caches when
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sending results as the dma based memory writes do no update eventually
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cached values on the cpu. now if there is no element next in the pipeline
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that actually reads from this memory area we could avoid the flushes. All
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other hardware elements and elements with any caps (tee, queue, capsfilter)
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are examples for those.
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