Historically, ADB transport protocol transfer speed was affected by two factors.
A_WRTE apacket was CRCed upon write and the CRC was checked upon read on the other end.A_WRTE apacket in-flight on an asocket. A local asocket would not schedule more data to be sent out until it had received an A_OKAY apacket response from its peer.The first issue was solved in aosp/568123. In that CL, the protocol was updated to remove the requirement for CRC generation and verification. This does not affect the reliability of a transport since both USB and TCP have packet checksums of their own.
The second issue is solved by "delayed ACK" (aosp/1953877), an experimental feature controlled by the environment variable ADB_BURST_MODE.
The idea is to introduce the concept of a per-asocket "available send bytes" (ASB) integer. This integer represent how many bytes we are willing to send without having received any A_OKAY for them.
While the ASB is positive, the asocket does not wait for an A_OKAY before sending more A_WRTE apackets. A remote asocket can be written to up until the ASB is exhausted.
The ASB capability is first negotiated on A_OPEN/A_OKAY exchange. After that, the ASB is maintained via decrement upon A_WRTE and increment upon A_OKAY.
This approach allows to "burst" A_WRTE packet but also "burst" A_OKAY packets to allow several A_WRTE packets to be in-flight on an asocket. This greatly increases data transfer throughput.
A_OPEN unused field (arg1) is repurposed to declare the wish to use delayed ACK features. If not supported, the receiving end of the A_OPEN will A_CLSE the connection.A_OKAY now has a payload (a int32_t) which acknowledge how much payload was received in the last received A_WRTE apacket.Here are two traces showing the timing of three A_WRTE.
Host > A_OPEN > Device Host > A_WRTE > Device The LS removes itself from the fdevent EPOLLIN and nothing is sent. Host < A_OKAY < Device The LS requests fdevent EPOLLIN for its fd to start reading and send more A_WRTE. Host > A_WRTE > Device The LS removes itself from the fdevent EPOLLIN and nothing is sent. Host < A_OKAY < Device The LS requests fdevent EPOLLIN for its fd to start reading and send more A_WRTE. Host > A_WRTE > Device The LS removes itself from the fdevent EPOLLIN and nothing is sent. Host < A_OKAY < Device The LS requests fdevent EPOLLIN for its fd to start reading and send more A_WRTE.
With ASB, see how A_WRTE and A_OKAY are burst instead of being paired.
Host(ASB=0) > A_OPEN(arg1=1MiB) > Device Host(ASB=X) < A_OKAY(<ASB=X>) < Device Host<ASB=X-a) > A_WRTE(payload size=a) > Device Host<ASB=Y-a-b) > A_WRTE(payload size=b) > Device Host<ASB=Z-a-b-c) > A_WRTE(payload size=c) > Device ASB is < 0. The LS removes itself from the fdevent EPOLLIN and nothing is sent. ... Host(ASB=X-b-c) < A_OKAY(<a>) < Device ASB is > 0. The LS requests fdevent EPOLLIN for its fd to start reading and send more A_WRTE. ... Host(ASB=X-c) < A_OKAY(<b>) < Device Host(ASB=X) < A_OKAY(<c>) < Device
Initial testing show that Burst Mode is nearly 70% faster at pushing files to a device over a USB-3 cable.
$ adb kill-server && unset ADB_BURST_MODE && adb start-server $ adb push -Z ~/Desktop/10G1 /data/local/tmp /usr/local/google/home/sanglardf/Desktop/10G1: 1 file pushed, 0 skipped. 202.0 MB/s (10737418240 bytes in 50.701s) $ adb push -Z ~/Desktop/10G1 /data/local/tmp /usr/local/google/home/sanglardf/Desktop/10G1: 1 file pushed, 0 skipped. 205.9 MB/s (10737418240 bytes in 49.724s) $ adb push -Z ~/Desktop/10G1 /data/local/tmp /usr/local/google/home/sanglardf/Desktop/10G1: 1 file pushed, 0 skipped. 197.6 MB/s (10737418240 bytes in 51.828s)
$ adb kill-server && export ADB_BURST_MODE=1 && adb start-server $ adb push -Z ~/Desktop/10G1 /data/local/tmp /usr/local/google/home/sanglardf/Desktop/10G1: 1 file pushed, 0 skipped. 337.2 MB/s (10737418240 bytes in 30.365s) $ adb push -Z ~/Desktop/10G1 /data/local/tmp /usr/local/google/home/sanglardf/Desktop/10G1: 1 file pushed, 0 skipped. 342.0 MB/s (10737418240 bytes in 29.945s) $ adb push -Z ~/Desktop/10G1 /data/local/tmp /usr/local/google/home/sanglardf/Desktop/10G1: 1 file pushed, 0 skipped. 341.3 MB/s (10737418240 bytes in 30.000s)