Re: 2.6.37: Multi-second I/O latency while untarring

[Matt]

On Fri, Feb 11, 2011 at 3:08 PM, Andrew Lutomirski <andy@xxxxxxx> wrote:
> As I type this, I have an ssh process running that's dumping data into
> a fifo at high speed (maybe 500Mbps) and a tar process that's
> untarring from the same fifo onto btrfs.  The btrfs fs is mounted -o
> space_cache,compress.  This machine has 8GB ram, 8 logical cores, and
> a fast (i7-2600) CPU, so it's not an issue with the machine struggling
> 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 = 1 on the disk (a slow 2TB WD) also makes
> no difference.
>
> max_sectors_kb = 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.  I 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?  I thought this stuff was supposed to be better on modern 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=y

Regards

Matt
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