Opened 2 months ago

Last modified 2 months ago

#33763 new enhancement

Consider setting TCP_NOTSENT_LOWAT to 1 byte on our TCP connections

Reported by: dgoulet Owned by:
Priority: Medium Milestone: Tor: 0.4.4.x-final
Component: Core Tor/Tor Version:
Severity: Normal Keywords: tor-relay performance scaling tcp
Cc: Actual Points:
Parent ID: Points: 0.2
Reviewer: Sponsor:


This TCP option would provide us and important performance benefit at the relay side.

In an effort to work on a new tor design to reduce kernel queuing delays, see it as an improvement to KIST and tor architecture, Rob and I submitted this paper to NetDev 0x14 conference in hope of sitting down with the Linux kernel developers and network experts about this idea.

Ultimately we want a new poll TCP option in the kernel which our experimentation shows how it improves performance drastically.

But turns out that starting with setting TCP_NOTSENT_LOWAT to 1 byte would significantly improve performance close to what we wanted. See Figure 1. (page 3) in the paper for the performance improvements.

Setting TCP_NOTSENT_LOWAT is minimal change to tor and thus we should do it.

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Change History (1)

comment:1 Changed 2 months ago by robgjansen

I thought this would be done after moving Tor to using epoll directly, so that we could stop using the part of KIST that checks for *tcp_info* every 10 milliseconds. Is that correct? Or do you envision using *TCP_NOTSENT_LOWAT* on top of KIST (which we never simulated/tested)?

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