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    SubjectRe: 2.6.37: Multi-second I/O latency while untarring
    Excerpts from Andrew Lutomirski's message of 2011-02-11 10:08:52 -0500:
    > 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.

    We can tell more if you post the full traces from latencytop. I have a
    patch here for latencytop that adds a -c mode, which dumps the traces
    out to a text files.

    Based on what you have here, I think it's probably a latency problem
    between btrfs and the dm-crypt stuff. How easily can setup a test
    partition without dm-crypt?


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