Am 06.12.2010 16:58, schrieb Nikos Chantziaras: > On 12/06/2010 04:33 PM, Stroller wrote: >> [...] >> >> The machine seems much snappier if I return to an interactive session >> after leaving it idle for some time, but during DVD rips it is still >> unresponsive for tens of seconds at a time. It seems like maybe it >> responds quicker than mainline sources, but it's still so slow that >> it's both unusable and hard to be sure whether that's the case. >>
As a side note: Have you tried using sys-process/latencytop to find the exact reason for your latency problem? >> Will try cgroups this week, perhaps. > > cgroups will not help with that either. I bet the problem is that the > kernel doesn't care about what kind of data it is caching. I came > across this on LKML, but no one was interested in a solution. > > The problem is that the kernel caches all data. Ripping a DVD will > consume 4GB cache, and all other data will be thrown out. Throwing that > all out again after the rip has finished is an awful lot of I/O load. > The smart thing to do would be for the kernel to not cache such stuff > (DVD rips, big video files, etc.) But it does. > > I fear I start to repeat myself, but I /think/ cgroups could help again. The memory subsystem allows you to set hard limits on the amount of memory used by a cgroup. This seems to include cache. Please take a look at [1] and [2]. Both should give you enough information to get you started. You should apply memory limits explicitly for certain processes/shells - not on a per-default basis as the CPU scheduling solution. Disclaimer: I have never tried this and do not intend to do this. [1] http://www.google.de/url?sa=t&source=web&cd=1&ved=0CBgQFjAA&url=http%3A%2F%2Fwww.linuxfoundation.jp%2Fjp_uploads%2Fseminar20081119%2FCgroupMemcgMaster.pdf&rct=j&q=%2Bcgroup%20%2B%22file%20cache%22&ei=ZFT9TNWtDZS08QOf-vihDA&usg=AFQjCNHd23FsqouJutTc4BAotkM9CUpfMQ&cad=rja [2] file:///usr/src/linux/Documentation/cgroups/memory.txt P.S.: We would not have this problem if application designers were more careful about i/o and cache performance. If the dvd ripper opened files with the option O_DIRECT (as specified in `man 2 open`), the cache would stay clean. Hope this helps, Florian Philipp
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