Re: [fuse-devel] read more than 128kB

[Miklos S. <mi...@sz...>]

> is there any reasonable way to increase the read size in FUSE from the 128kB limit?
> 
> Right now FUSE or the kernel split all >128kB user read()'s into several 
> <=128kB read calls, this severely reduces performance. There's a data 
> acquisition DAQ card I'm reading from.
> 
> 
> With a simple non-FUSE test program that does >4 MB reads through the DAQ 
> cards own kernel module API, I get around 1500 Mbit/s throughput.
> 
> But with the FUSEs chunked 128kB reads, performance is around 30 Mbit/s. 
> That's lousy.
> 
> I then added some buffering/caching code to my FUSE read(). It does one 
> blocking DAQ card read of 16MB and then memcpy()s data out to the user on 
> each next read() call. Performance now is ~400 Mbit/s. But significant 
> time is lost in the memcpy() - which would be unnecessary if FUSE could 
> pass larger than 128kB data out to the user.

Hmm, what is the performance if you just comment out the memcpy()?

> So is there any way (no custom kernels thanks!) to get better performance 
> out of FUSE? Modify FUSE code? And increase the maximum read size from 
> measly 128kB to a few megabytes somehow?

It should be possible if you increase FUSE_MAX_PAGES_PER_REQ from 32
to 1024 in fuse/fuse_i.h in the kernel tree, and give
"-omax_read=4194304" mount option.

I'm curious how much that helps.

Miklos