stagefright: httplive: Decouple block size from bandwidth estimate

A very small block size in PlaylistFetcher can lead to framework
overhead and difficulty streaming high bitrate content, but since
HTTPBase keeps a constant history of the past 100 HTTP reads, the
block size directly affects bandwidth estimation and in turn,
switching latency.

Add setBandwidthHistorySize() to HTTPBase to allow setting the
history size for bandwidth estimation. Call this within LiveSession
based on the current block size to ensure that the number of bytes
used for estimating bandwidth does not change if the block size is
changed in PlaylistFetcher.

Since a single TCP/IP packet can contain up to 64k of data, increase
the block size in PlaylistFetcher from 2k to lcm(188, 1024) or 47k to
avoid inaccuracies in read timings due to up to a comparable 47 reads
from the same locally-cached packet instead of from the network.

Also make HTTPBase::addBandwidthMeasurement() virtual to allow
bandwidth estimation extensions that do not rely on a history list.

Bug: 18821145
Change-Id: I5f957be01f5346e74cfb7eeb150ca4b397ad5798

1 files changed, 7 insertions, 2 deletions

diff --git a/media/libstagefright/HTTPBase.cpp b/media/libstagefright/HTTPBase.cpp
index 32291c8..0c2ff15 100644
--- a/media/libstagefright/HTTPBase.cpp
+++ b/media/libstagefright/HTTPBase.cpp
@@ -36,7 +36,8 @@ HTTPBase::HTTPBase()
       mTotalTransferBytes(0),
       mPrevBandwidthMeasureTimeUs(0),
       mPrevEstimatedBandWidthKbps(0),
-      mBandWidthCollectFreqMs(5000) {
+      mBandWidthCollectFreqMs(5000),
+      mMaxBandwidthHistoryItems(100) {
 }
 
 void HTTPBase::addBandwidthMeasurement(
@@ -50,7 +51,7 @@ void HTTPBase::addBandwidthMeasurement(
     mTotalTransferBytes += numBytes;
 
     mBandwidthHistory.push_back(entry);
-    if (++mNumBandwidthHistoryItems > 100) {
+    if (++mNumBandwidthHistoryItems > mMaxBandwidthHistoryItems) {
         BandwidthEntry *entry = &*mBandwidthHistory.begin();
         mTotalTransferTimeUs -= entry->mDelayUs;
         mTotalTransferBytes -= entry->mNumBytes;
@@ -104,6 +105,10 @@ status_t HTTPBase::setBandwidthStatCollectFreq(int32_t freqMs) {
     return OK;
 }
 
+void HTTPBase::setBandwidthHistorySize(size_t numHistoryItems) {
+    mMaxBandwidthHistoryItems = numHistoryItems;
+}
+
 // static
 void HTTPBase::RegisterSocketUserTag(int sockfd, uid_t uid, uint32_t kTag) {
     int res = qtaguid_tagSocket(sockfd, kTag, uid);