Why is the quiche timer still used after using SO_TXTIME?

[bianmingkun]

In TCP, pacing is changed from the internal timer of the TCP protocol stack to the sch_fq of qdisc（EDT），which can reduce the load of the timer.

In UDP(quic), quic supports SO_TXTIME after the following patch. It can use sch_fq of qdisc to accurately control the sending time like TCP's fq.
acfdb39#diff-efe9b159470f6628daa390488bfa556bc44dae31c89682ae90554ff9b6fe7993R87

At the same time, this patch can more accurately determine the packet sending time, which can make the calculation of rtt more accurate by function "QuicGsoBatchWriter::GetReleaseTime".

My question is:

1. Why does quiche still need a timer (TCP has deleted the pacing timer) Since SO_TXTIME is used, Is it because UDP does not have a TSQ mechanism similar to TCP（This mechanism prevents the qdisc queue from being stored in too many skb）？

2. Can I just use the release_time mechanism, not use SO_TXTIME? I think release_time is more for calculating a more accurate RTT.
   For example， Only Delele “*hdr->GetNextCmsgData<uint64_t>(SOL_SOCKET, SO_TXTIME) = release_time;” In "QuicGsoBatchWriter::BuildCmsg".