Slow swapping even on fast infiniband

[Spelic <spelic@shiftmail.org>]

Hello all lkml,

I have just set up two servers connected through iSCSI over Infiniband 
(SCST / SRP).

The "target" end exposes a ramdisk over SRP, the "initiator" end uses 
such device as a swap.
(I am trying to aggregate the memory of a few computers in order to 
perform computations not possible with the RAM of one only.)

This remote SRP disk is very fast, around 1 GByte/sec if I write or read 
to it using dd at bs=4K; from the initiator computer. So the IB is not 
the bottleneck.

However if I use such disk as a swap device on the "initiator" computer, 
I seem not able to obtain more than 150MB/sec reads + 150MB/sec writes 
from/to the swap
I can see these figures with iostat and I can roughly confirm them by 
the time it takes for my C++ memory-sweep-test to sweep all the RAM+swap 
for a few rounds.

Why kswapd is so slow?
Is there a way to do faster swapping of pages, such as with some kind of 
readahead or somehow swapping larger chunks together...?

I tweaked lots of settings in /sys/block/sdc/queue/  (scheduler, 
nr_requests, queue_depth), in the /proc/sys/vm/ (dirty_ratio, background 
ratio etc) but 150MB/sec is the most I could obtain.
Remember that this disk performs almost 1GB/sec in write and read tests 
with dd bs=4K. (The srp disk is being used as full device: no 
partitions, no LVM, no RAID.)
Will linux never be able to swap faster than this?

My kernel is 2.6.32 with just a few patches from the scst people.

Thanks for your help
Spelic
PS: please possibly keep me in CC if you reply because I am not 
subscribed to lkml. Ok I will also check via web. Thank you