Re: Slow file transfer speeds with CFQ IO scheduler in some cases

[Wu Fengguang]

On Thu, Feb 12, 2009 at 09:35:18PM +0300, Vladislav Bolkhovitin wrote:
> Sorry for such a huge delay. There were many other activities I had to  
> do before + I had to be sure I didn't miss anything.
>
> We didn't use NFS, we used SCST (http://scst.sourceforge.net) with  
> iSCSI-SCST target driver. It has similar to NFS architecture, where N  
> threads (N=5 in this case) handle IO from remote initiators (clients)  
> coming from wire using iSCSI protocol. In addition, SCST has patch  
> called export_alloc_io_context (see  
> http://lkml.org/lkml/2008/12/10/282), which allows for the IO threads  
> queue IO using single IO context, so we can see if context RA can  
> replace grouping IO threads in single IO context.
>
> Unfortunately, the results are negative. We find neither any advantages  
> of context RA over current RA implementation, nor possibility for  
> context RA to replace grouping IO threads in single IO context.
>
> Setup on the target (server) was the following. 2 SATA drives grouped in  
> md RAID-0 with average local read throughput ~120MB/s ("dd if=/dev/zero  
> of=/dev/md0 bs=1M count=20000" outputs "20971520000 bytes (21 GB)  
> copied, 177,742 s, 118 MB/s"). The md device was partitioned on 3  
> partitions. The first partition was 10% of space in the beginning of the  
> device, the last partition was 10% of space in the end of the device,  
> the middle one was the rest in the middle of the space them. Then the  
> first and the last partitions were exported to the initiator (client).  
> They were /dev/sdb and /dev/sdc on it correspondingly.

Vladislav, Thank you for the benchmarks! I'm very interested in
optimizing your workload and figuring out what happens underneath.

Are the client and server two standalone boxes connected by GBE?

When you set readahead sizes in the benchmarks, you are setting them
in the server side? I.e. "linux-4dtq" is the SCST server? What's the
client side readahead size?

It would help a lot to debug readahead if you can provide the
server side readahead stats and trace log for the worst case.
This will automatically answer the above questions as well as disclose
the micro-behavior of readahead:

        mount -t debugfs none /sys/kernel/debug

        echo > /sys/kernel/debug/readahead/stats # reset counters
        # do benchmark
        cat /sys/kernel/debug/readahead/stats

        echo 1 > /sys/kernel/debug/readahead/trace_enable
        # do micro-benchmark, i.e. run the same benchmark for a short time
        echo 0 > /sys/kernel/debug/readahead/trace_enable
        dmesg

The above readahead trace should help find out how the client side
sequential reads convert into server side random reads, and how we can
prevent that.

Thanks,
Fengguang
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