Re: BerkeleyDB performance on Linux

[Howard Chu <hyc@symas.com>]

Rick Jones wrote:
> Howard Chu wrote:
> > The attached patch makes O_DIRECT work on Linux in BerkeleyDB 4.5.20. 
> > (You will need to manually define LINUX_NEEDS_PAGE_ALIGNMENT if you're 
> > using a kernel older than 2.6.)
> >
> > The main reason to use this patch is to conserve memory - ordinarily, 
> > all the I/O that BDB does to its files gets cached in the Linux 
> > filesystem buffer cache. This caching is redundant since BDB always 
> > does its own caching, and it effectively makes the BDB environment 
> > consume twice as much memory as it needs. Using O_DIRECT on I/Os 
> > disables the filesystem buffer cache for those I/Os, thus freeing up a 
> > sizable chunk of memory.
>
> Long ago and far away I was having "fun" with SPECweb99 on Linux and 
> found that eventually the access logs would grow and cast-out otherwise 
> usefully cached URLs.  I then found the posix_fadvise call, which can be 
> used to tell the system that a certain range is no longer required:

Yes, but posix_fadvise is not supported on Linux kernels before 2.6, returns 
ENOSYS.

--
  -- Howard Chu
  Chief Architect, Symas Corp.  http://www.symas.com
  Director, Highland Sun        http://highlandsun.com/hyc
  OpenLDAP Core Team            http://www.openldap.org/project/