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. Erwan, -- 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/
