* Sebastian Reitenbach <[email protected]> [2011-12-03 01:12]: > Thanks to your pointers on icb, I found the autosizing algorithm at the end > of netinet/tcp_userreg.c. > There I saw its comparing the actual value of fill level of the buffer, and > how much got transferred against a > maximum defined in sys/socketvar.h: > > #define SB_MAX (256*1024) > > With the default value of 256*1024, I got the maximum transfer rate of about > 1.5MB/s. > This speed there was fairly constant, when the maximum was reached. > Then I doubled the value to 512*1024, and got constant transfer rates of > about 2.7MB/s, which is about the same speed like I got with the Linux host. > Then I again doubled the value to 1024*1024, and got speeds of about 5MB/s. > This was not so constant anymore. Here I got spikes of up to 7MB/s, and in > the middle it dropped down to 2-3 MB/s. > Then again doubled the value to 2048*1024, and then I got transfer speeds of > about 8MB/s. So about 2.5 times faster than with Linux as the server. Also > here, the transfer rates are not constant over the download, they are varying > from 3MB/s and going up to over 9MB/s, nearly to the maximum the network card > in the server provides (100MBit).
ok, so we're dealing with a high-latency high-bandwidth connection. there bigger buffers and thus seqnr windows help a lot - due to the latency. > Since my knowledge to the network stack is next to zero, I don't really know > what side effects it would have, to raise the default to a higher value, and > if it would acceptable to have those varying transfer speeds, instead of the > constant rates with the lower values. > Maybe a problem for machines with lower memory? "low memory" is a bit misleading, we're talking about pretty special memory here - but yes, hundreds or thousands of sockets with 2MB buffers in use each would be a problem. there is a backpressure mechanism in the autosizing algorithm to (try to) avoid this; we might very well want to increase SB_MAX a bit. -- Henning Brauer, [email protected], [email protected] BS Web Services, http://bsws.de, Full-Service ISP Secure Hosting, Mail and DNS Services. Dedicated Servers, Root to Fully Managed Henning Brauer Consulting, http://henningbrauer.com/
