Alan Stange wrote: > Bruce Momjian wrote: > > Right now the file system will do read-ahead for a heap scan (but not an > > index scan), but even then, there is time required to get that kernel > > block into the PostgreSQL shared buffers, backing up Luke's observation > > of heavy memcpy() usage. > > > > So what are our options? mmap()? I have no idea. Seems larger page > > size does help.
> For sequential scans, you do have a background reader. It's the > kernel. As long as you don't issue a seek() between read() calls, the I guess you missed my text of "Right now the file system will do read-ahead", meaning the kernel. > I don't think the memcpy of data from the kernel to userspace is that > big of an issue right now. dd and all the high end network interfaces > manage OK doing it, so I'd expect postgresql to do all right with it now > yet too. Direct IO will avoid that memcpy, but then you also don't get > any caching of the files in memory. I'd be more concerned about any > memcpy calls or general data management within postgresql. Does > postgresql use the platform specific memcpy() in libc? Some care might > be needed to ensure that the memory blocks within postgresql are all > properly aligned to make sure that one isn't ping-ponging cache lines > around (usually done by padding the buffer sizes by an extra 32 bytes or > L1 line size). Whatever you do, all the usual high performance > computing tricks should be used prior to considering any rewriting of > major code sections. We have dealt with alignment and MemCpy is what we used for small-sized copies to reduce function call overhead. If you want to improve it, feel free to take a look. -- Bruce Momjian | http://candle.pha.pa.us pgman@candle.pha.pa.us | (610) 359-1001 + If your life is a hard drive, | 13 Roberts Road + Christ can be your backup. | Newtown Square, Pennsylvania 19073 ---------------------------(end of broadcast)--------------------------- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
