On Fri, Feb 11, 2011 at 3:08 PM, Andrew Lutomirski [off-list ref] wrote=
:

As I type this, I have an ssh process running that's dumping data int=

o

a fifo at high speed (maybe 500Mbps) and a tar process that's
untarring from the same fifo onto btrfs. =A0The btrfs fs is mounted -=

o

space_cache,compress. =A0This machine has 8GB ram, 8 logical cores, a=

nd

a fast (i7-2600) CPU, so it's not an issue with the machine strugglin=

g

under load.

Every few tens of seconds, my system stalls for several seconds.
These stalls cause keyboard input to be lost, firefox to hang, etc.

Setting tar's ionice priority to best effort / 7 or to idle makes no =

difference.

ionice idle and queue_depth =3D 1 on the disk (a slow 2TB WD) also ma=

kes

no difference.

max_sectors_kb =3D 64 in addition to the above doesn't help either.

latencytop shows regular instances of 2-7 *second* latency, variously
in sync_page, start_transaction, btrfs_start_ordered_extent, and
do_get_write_access (from jbd2 on my ext4 root partition).

echo 3 >drop_caches gave me 7 GB free RAM. =A0I still had stalls when
4-5 GB were still free (so it shouldn't be a problem with important
pages being evicted).

In case it matters, all of my partitions are on LVM on dm-crypt, but
this machine has AES-NI so the overhead from that should be minimal.
In fact, overall CPU usage is only about 10%.

What gives? =A0I thought this stuff was supposed to be better on mode=

rn kernels.

--Andy
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Hi Andrew,

you could try the following patch to speed up dm-crypt:

https://patchwork.kernel.org/patch/365542/

I'm using it on top of a highly-patched 2.6.37 kernel

not sure if exactly that version was included in 2.6.38


there are some additional handles to speed up dm:

e.g. PCRYCONFIG_CRYPTO_PCRYPT=3Dy

Regards

Matt