I am a newbie of QUIC protocol. Now that chrome has enabled supporting gQUIC be default, I want to capture some packets to check. Surprisingly, I opened wireshark, set gquic || quic filter, and tried to access youtube or google, nothing was displayed. But in fact, there were some packets which used gQUIC protocols, i.e., there is “Q050” in following diagram:
I checked wireshark code base, unfortunately, wireshark only supports gQUIC up to Q043(please refer this commit).
Yesterday, I did performance analysis for one project. This is the output of mpstat command for old version:
And this is the CPU utilisation for new version:
For new version, the iowait ratio is remarkably high. After Checking the code, I found the original serialisation was just a fflush, but now for some reasons, it was replaced by fdatasync which is a blocking API and only returns when the data is transferred to the storage device. Therefore the thread which invokes fdatasync will be stuck there and can’t process any other message. So we must pay attention to use blocking IO APIs, sometimes they may bring you results which you don’t want.