dccd tune-up

Vernon Schryver vjs@calcite.rhyolite.com
Fri Jun 6 14:06:59 UTC 2003

> From: Valentin Chopov <valentin@valcho.net>

> I several times tried --with-db-memory=X and --enable-big-db configuration
> parameters and I couldn't see any difference in the dccd performance.
> Is there any reason to change them and when? 

with-db-memory is useful only on systems with plenty of memory 
or where getrlimit() does not tell a program how much memory is available.
Dccd tries to use half of available physical memory.  A system with
lots, that runs little besides dccd, and where dccd is busy can benefit
from a with-db-memory setting.

enable-big-db increases the size of pointers in the database hash table.
The default allows only 16 million different checksums in the database.
The larger pointers allow 2 billion checksums, but also make each
entry larger and so makes the hash table itself larger.  A larger hash
table will have a larger memory footprint and be slower.

>                                              And my main problem are RTTs,
> I still getting high RTTs and I can't understand why? BTW, all my servers
> are FreeBSD.

The RTT for a local DCC operation should be microseconds unless the
network or operating system imposes delays or a lot of requests arrive
at once.  Operating system delays are caused by either other applications
that need to run or by paging and swapping when the database won't
fit in physical memory or the operating system does something stupid
with mapped files (e.g. BSD/OS and some flavors of Linux).

Vernon Schryver    vjs@rhyolite.com

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