On Monday 22 August 2011 20:12:13 Miroslav Ľos wrote: > > On Aug. 21, 2011, 10:07 a.m., David Faure wrote: > > > Thanks Peter and Miroslav. The analysis looks correct, the pre-read part > > > of the patch looks good. I'm just wondering about using Unbuffered. If > > > someone installs a mimetype definition with multiple rules trying to > > > match some bytes after the 2K limit, then all this seeking-and-reading > > > back and forth will be very slow, in unbuffered mode (since neither > > > cache will be used). > I find Unbuffered causing slowness improbable. From what I've seen in uses > of KMimeType::findBy\w*Content in kdelibs, they all (e.g. ftp* and http > kioslaves) rather provide their own buffer in a QByteArray rather than a > QIODevice; most just provide the path (which is only opened using QFile if > it is_local_file).
The most common case for mimetype determination is from the file manager listing local files, in which case findByUrl will use a QFile for content- determination. Ah, and BTW I have just found a magic rule that needs 4K of data: vmware-player.xml says: <match type="string" value='config.version = "' offset="0:4096"/> > All QFile's buffering is implemented in the QIODevice superclass, it adds > Unbuffered in open() to its openMode for its fileEngine() backend. Thus, no > buffering is propagated down. I'm not sure what you mean there. > The unnecessary 16K read did hit several more > EIO's on the broken CD I have, but it is just cosmetic I guess. Yes I'm not sure it's worth optimizing for this special case. > Nonetheless, I find most uses of these functions ever will be through QFile > or QBuffer Sure. It's QFile that I have in mind here, when I say that seeking back and forth will be slow, in unbuffered mode. > Finally, I wonder if buffering makes a difference as we are only using a few > small block reads, not many getChar()'s. It's about seeking and reading, vs just having the data in memory. > * ftp passes a 1K-capped buffer (kioslave/ftp/ftp.cpp:2471). That may be > insufficient for the single rule on my system that needed 1029 bytes. Right. Small bug, but a corner case (only matters if extension unknown). -- David Faure, fa...@kde.org, http://www.davidfaure.fr Sponsored by Nokia to work on KDE, incl. Konqueror (http://www.konqueror.org).
