On Monday 29 March 2010 16:32:08 you wrote: > [CC to d...@apr.apache.org] > > From: Stefan Fuhrmann [mailto:stefanfuhrm...@alice-dsl.de] > > Under Windows, a significant part of the overall runtime > > is spent in writing data to the (already buffered) APR > > file implementation. It seems that the mutex serializing > > the buffer access in apr_file_write is expensive. > > > > Also, >50% of all write requests are 2 bytes or smaller > > (i.e. line endings and empty lines). For them, the deep > > call hierarchy constitutes a large overhead on register- > > lacking x86. > > > > This patch eliminates far over 90% of all write requests > > bringing the portion of time spent in _svn_io_file_write_full > > down from about 7 to below 3 percent on 32 bit Windows. > > > > Performance gain is ~1% under Linux but due to the > > larger async I/O and mutex overhead it is about 4% > > under Windows: > > Hi Stefan, > > Thanks for looking into this. > > I'm just wondering why APR does use the mutexes even though APR_XTHREAD > isn't passed? > Maybe because the mutex APR uses, a critical section, is more costly now > than when this code was implemented several years ago? > (On a singlecore machine a simple interlocked increment is very cheap, but > on a multicore machine it requires at least some synchronization). > > Looking at the APR help it might be better to fix the buffering in APR, > than to add yet another level of buffering. (Our buffer, APR's buffering > and then the OS buffering). > > After a quick search through the apr code it seems that only windows tries > to make file writes always thread safe, while other operating systems use > APR_XTHREAD as a trigger. > > > The Windows code also enables overlapped IO on APR_XTHREAD, so there could > be some valid use where you want thread safe code, but not overlapped. But > why only do this on Windows? (Maybe the APR team can answer this question).
Thank you for the feedback. I will try to come up with a patch that reduces the APR file access overhead. Depending on the results, I would then either send a patch to the APR team or continue the discussion here. -- Stefan^2.
