On Wed, 07 Nov 2012 21:48:17 +0100 Erwan Velu <erwanalia...@gmail.com> wrote:
> Hi fellows, > > I'm been facing some lseek() troubles on a very light hardware (Atom E660) > under heavy load (network + cpu + disk IOs). I'm using 3.2.32 on a 32bit Os > with a local SSD as mass storage. > > If a do open a block device like sdb1 and lseek SEEK_SET in it, some > unexpected latencies occurs. > Using the same load, everything works perfectly by using contigous streams > but once I do lseek it start to be laggy. I've been searching around for a > while and finally found this message : https://lkml.org/lkml/2011/9/15/399 > from Andy. > > The description was very similar to what I experienced but the patch from > Andy was on to the fs layer. > > I've been looking the code for the block level layer and found the > implementation is pretty different. > http://lxr.linux.no/#linux+v3.2.33/fs/read_write.c#L69 > vs > http://lxr.linux.no/#linux+v3.2.33/fs/block_dev.c#L353 > > As I can see, we do first put the mutex, then i_size_read and then > considering the kind of SEEK we want. > The semantic changes from the read_write implementation where it does the > locking only for SEEK_CUR and i_size_read isn't executed for SEEK_SET. > > So I really wonder if we shall rework this part to avoid the uncessary > locking for all of them except SEEK_CUR and remove i_size_read from SEEK_SET. > The i_size_read is also a matter as it does a memory barrier. On such low-end > hardware I have, that could costs. > > I can work on it and validate its performances unless the experts you are > told me this is a mandatory feature. > > Thanks for your attention and comments on this topic. If your lseek()ing process is indeed blocking on i_mutex then something else must be holding it. ie: there's some heavy I/O happening against that device at the same time? Tell us more... Another possibility is that the delay is not in lseek() but is actually in the device open/close, doing lots of pagecache invalidation and/or writeout. It used to be the case that blockdev close() would do a heavyweight flush/invalidate, but I haven't checked lately. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
