Unexpected latencies on lseek() SEEK_SET on block devices

[Erwan Velu]

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,
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