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| -rw-r--r-- | wiki/dev/buffers.md | 54 |
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diff --git a/wiki/dev/buffers.md b/wiki/dev/buffers.md index eef01aa..62bcad6 100644 --- a/wiki/dev/buffers.md +++ b/wiki/dev/buffers.md @@ -17,6 +17,49 @@ seconds, not in bytes**, and managed accordingly. Less obviously, buffer management needs to be considerably more sophisticated than the usual "grow buffer when full, up to some predefined maximum size." +Point one also implies a rule that I see honoured more in ignorance than in +awareness: **you can't make a full buffer less full by making it bigger**. Size +is not a factor in buffer fullness, only in buffer latency, so adjusting the +size in response to capacity pressure is worse than useless. + +There are only three ways to make a full buffer less full: + +1. Increase the rate at which data exits the buffer. + +2. Slow the rate at which data enters the buffer. + +3. Evict some data from the buffer. + +In actual practice, most full buffers are upstream of some process that's +already going as fast as it can, either because of other design limits or +because of physics. A buffer ahead of disk writing can't drain faster than the +disk can accept data, for example. That leaves options two and three. + +Slowing the rate of arrival usually implies some variety of _back-pressure_ on +the source of the data, to allow upstream processes to match rates with +downstream processes. Over-large buffers delay this process by hiding +back-pressure, and buffer growth will make this problem worse. Often, +back-pressure can happen automatically: failing to read from a socket, for +example, will cause the underlying TCP stack to apply back-pressure to the peer +writing to the socket by delaying TCP-level message acknowledgement. Too often, +I've seen code attempt to suppress these natural forms of back-pressure without +replacing them with anything, leading to systems that fail by surprise when +some other resource – usually memory – runs out. + +Eviction relies on the surrounding environment, and must be part of the +protocol design. Surprisingly, most modern application protocols get very +unhappy when you throw their data away: the network age has not, sadly, brought +about protocols and formats particularly well-designed for distribution. + +If neither back-pressure nor eviction are available, the remaining option is to +fail: either to start dropping data unpredictably, or to cease processing data +entirely as a result of some resource or another running out, or to induce so +much latency that the data is useless by the time it arrives. + +----- + +Some uncategorized thoughts: + * Some buffers exist to trade latency against the overhead of coordination. A small buffer in this role will impose more coordination overhead; a large buffer will impose more latency. @@ -28,6 +71,10 @@ buffer when full, up to some predefined maximum size." an inordinate proportion of latency and throughput negotiating buffer sizes and message readiness. + * A coordination buffer is most useful when _empty_; in the ideal case, the + buffer is large enough to absorb one message's worth of data from the + source, then pass it along to the sink as quickly as possible. + * Some buffers exist to trade latency against jitter. A small buffer in this role will expose more jitter to the upstream process. A large buffer in this role will impose more latency. @@ -43,3 +90,10 @@ buffer when full, up to some predefined maximum size." down, causing them to under-deliver if the buffer drains, pushing the system back to a high-throughput mode. [This problem gets worse the more buffers are present in a system](http://www.bufferbloat.net). + + * An anti-jitter buffer is most useful when _full_; in exchange for a + latency penalty, sudden changes in throughput will be absorbed by data + in the buffer rather than propagating through to the source or sink. + +* Multimedia people understand this stuff at a deep level. Listen to them when + designing buffers for other applications. |