From: Chuck Lever Date: Thu, 08 Sep 2005 18:16:06 GMT Subject: Re: [PATCH 0/2] A new merge algorithm, take 3 Message-ID: <43207FE6.2030108@citi.umich.edu> In-Reply-To: Linus Torvalds wrote: > > On Thu, 8 Sep 2005, Junio C Hamano wrote: > >>Yes, the reading of three trees upfront is probably the culprit >>in your case > > > However, note that _most_ tree reading just reads one. > > Merges may take half a second, and yes, when I did it, the fact that we > move things around in the array is by far the highest cost. But the thing > is, if merges take half a second, that's still not only damn good, it's > not even the most common operation. in my case the merges were taking significantly longer than a half second. making this change is certainly not worth it if merges are running fast... > Yes, the active_cache[] layout as one big array is inconvenient for > read_tree(), which tends to want to interleave different trees in the > array and thus forces things to be moved around. > > But remember what the most common use for the index is - it's the "single > tree" case from read_cache(). That's _so_ much more common than > read_tree() that it's not even funny. read_cache is fast as implemented. the issue is that the next thing that is often done is a cache insert, which requires a memmove, which is slow. > So the data structure is optimized for a different case than reading in > trees. Big deal. That optimization is definitely worth it: it allows us to > do the read_cache() with the actual index entries being totally read-only > (a linked list would have to add a "next" pointer to the cache entries and > not allow the in-place thing that read_cache() does). they are still read-only with my linked list implementation. begin:vcard fn:Chuck Lever n:Lever;Charles org:Network Appliance, Incorporated;Linux NFS Client Development adr:535 West William Street, Suite 3100;;Center for Information Technology Integration;Ann Arbor;MI;48103-4943;USA email;internet:cel@citi.umich.edu title:Member of Technical Staff tel;work:+1 734 763-4415 tel;fax:+1 734 763 4434 tel;home:+1 734 668-1089 x-mozilla-html:FALSE url:http://www.monkey.org/~cel/ version:2.1 end:vcard