Re: SMP on FreeBSD 6.x and 7.0: Worth doing?

On Saturday 22 December 2007, Brett Glass wrote:
I will need to build several Web caches over the next few months,
and just took advantage of the Christmas lull (and a snowy day,
when I couldn't work outside) to test FreeBSD 7.0 BETA 4 to see how
it will perform at this task. I built up a 4 core FreeBSD box, and
asked a friend who's a Linux fanatic to do the same with Linux on
identical hardware. I didn't watch closely how he installed
everything, but asked him not to tune it beyond setting it up
properly for SMP.

We then ran a test suite in which a client starts several
processes. Each uses wget to fetch a series of objects in rapid
succession via the cache. The fetches done by each process are the
same batch of URLS, but shuffled differently, so each URL will get
a miss the first time and then hits each time it comes up
thereafter unless the cache overflows. We're doing all GETs, with
no tricky stuff like subranges.

As has been reported in some other messages on this list, Linux is
currently blowing FreeBSD away. It's taking as much as 20% less
time to get through the benchmark, depending on exactly how the
random shuffle came out. This is with 4 GB RAM, the GENERIC FreeBSD
SMP kernel (using SCHED_ULE), and aufs as the storage schema for Squid.

It appears, though I'd need to instrument the code more to be sure,
that the slowdown is coming from file I/O. Could it be that there
less concurrency or more overhead in FreeBSD file operations than
there is in Linux? Even with SoftUpdates turned on, the cache
volume mounted with -noatime, and aufs (which uses kqueues -- a
FreeBSD invention -- to optimize multithreaded disk access), the
benchmark shows FreeBSD losing out. Why?

Since you're using the fs as a cache, I presume it wouldn't be a big problem
if the data was lost by a power outage (or crash). If so, you can try the
async mount option to seriously increase fs performance.

Pieter de Goeje
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