Re: [RFC v3 0/4] vfs: Non-blockling buffered fs read (page cache only)

[Milosz Tanski]

I haven't seen too many replies to this last request so I imagine
there's not to many things to fix.. which is good to see.

Additionally, with Jeff's help I wrote up man pages to be ready to go
with these changes.

I'm thinking of changing the preadv2 call to be a preadv6/pwritev6
call for the next submission... so the number represents the argument
count. This should more closely follow the other syscall convention
(like accept4, wait4, dup3).

I'm going to be away for a week. I hope to have the next version (with
the above change) for you guys to review next Wednesday.

Best,
- Milosz

On Wed, Sep 24, 2014 at 5:46 PM, Milosz Tanski <milosz@xxxxxxxxx> wrote:
> This patcheset introduces an ability to perform a non-blocking read from
> regular files in buffered IO mode. This works by only for those filesystems
> that have data in the page cache.
>
> It does this by introducing new syscalls new syscalls preadv2/pwritev2. These
> new syscalls behave like the network sendmsg, recvmsg syscalls that accept an
> extra flag argument (RWF_NONBLOCK).
>
> It's a very common patern today (samba, libuv, etc..) use a large threadpool to
> perform buffered IO operations. They submit the work form another thread
> that performs network IO and epoll or other threads that perform CPU work. This
> leads to increased latency for processing, esp. in the case of data that's
> already cached in the page cache.
>
> With the new interface the applications will now be able to fetch the data in
> their network / cpu bound thread(s) and only defer to a threadpool if it's not
> there. In our own application (VLDB) we've observed a decrease in latency for
> "fast" request by avoiding unnecessary queuing and having to swap out current
> tasks in IO bound work threads.
>
>
> Version 3 highlights:
>  - Down to 2 syscalls from 4; can user fp or argument position.
>  - RWF_NONBLOCK value flag is not the same O_NONBLOCK, per Jeff.
>
> Version 2 highlights:
>  - Put the flags argument into kiocb (less noise), per. Al Viro
>  - O_DIRECT checking early in the process, per. Jeff Moyer
>  - Resolved duplicate (c&p) code in syscall code, per. Jeff
>  - Included perf data in thread cover letter, per. Jeff
>  - Created a new flag (not O_NONBLOCK) for readv2, perf Jeff
>
>
> Some perf data generated using fio comparing the posix aio engine to a version
> of the posix AIO engine that attempts to performs "fast" reads before
> submitting the operations to the queue. This workflow is on ext4 partition on
> raid0 (test / build-rig.) Simulating our database access patern workload using
> 16kb read accesses. Our database uses a home-spun posix aio like queue (samba
> does the same thing.)
>
> f1: ~73% rand read over mostly cached data (zipf med-size dataset)
> f2: ~18% rand read over mostly un-cached data (uniform large-dataset)
> f3: ~9% seq-read over large dataset
>
> before:
>
> f1:
>     bw (KB  /s): min=   11, max= 9088, per=0.56%, avg=969.54, stdev=827.99
>     lat (msec) : 50=0.01%, 100=1.06%, 250=5.88%, 500=4.08%, 750=12.48%
>     lat (msec) : 1000=17.27%, 2000=49.86%, >=2000=9.42%
> f2:
>     bw (KB  /s): min=    2, max= 1882, per=0.16%, avg=273.28, stdev=220.26
>     lat (msec) : 250=5.65%, 500=3.31%, 750=15.64%, 1000=24.59%, 2000=46.56%
>     lat (msec) : >=2000=4.33%
> f3:
>     bw (KB  /s): min=    0, max=265568, per=99.95%, avg=174575.10,
>                  stdev=34526.89
>     lat (usec) : 2=0.01%, 4=0.01%, 10=0.02%, 20=0.27%, 50=10.82%
>     lat (usec) : 100=50.34%, 250=5.05%, 500=7.12%, 750=6.60%, 1000=4.55%
>     lat (msec) : 2=8.73%, 4=3.49%, 10=1.83%, 20=0.89%, 50=0.22%
>     lat (msec) : 100=0.05%, 250=0.02%, 500=0.01%
> total:
>    READ: io=102365MB, aggrb=174669KB/s, minb=240KB/s, maxb=173599KB/s,
>          mint=600001msec, maxt=600113msec
>
> after (with fast read using preadv2 before submit):
>
> f1:
>     bw (KB  /s): min=    3, max=14897, per=1.28%, avg=2276.69, stdev=2930.39
>     lat (usec) : 2=70.63%, 4=0.01%
>     lat (msec) : 250=0.20%, 500=2.26%, 750=1.18%, 2000=0.22%, >=2000=25.53%
> f2:
>     bw (KB  /s): min=    2, max= 2362, per=0.14%, avg=249.83, stdev=222.00
>     lat (msec) : 250=6.35%, 500=1.78%, 750=9.29%, 1000=20.49%, 2000=52.18%
>     lat (msec) : >=2000=9.99%
> f3:
>     bw (KB  /s): min=    1, max=245448, per=100.00%, avg=177366.50,
>                  stdev=35995.60
>     lat (usec) : 2=64.04%, 4=0.01%, 10=0.01%, 20=0.06%, 50=0.43%
>     lat (usec) : 100=0.20%, 250=1.27%, 500=2.93%, 750=3.93%, 1000=7.35%
>     lat (msec) : 2=14.27%, 4=2.88%, 10=1.54%, 20=0.81%, 50=0.22%
>     lat (msec) : 100=0.05%, 250=0.02%
> total:
>    READ: io=103941MB, aggrb=177339KB/s, minb=213KB/s, maxb=176375KB/s,
>          mint=600020msec, maxt=600178msec
>
> Interpreting the results you can see total bandwidth stays the same but overall
> request latency is decreased in f1 (random, mostly cached) and f3 (sequential)
> workloads. There is a slight bump in latency for since it's random data that's
> unlikely to be cached but we're always trying "fast read".
>
> In our application we have starting keeping track of "fast read" hits/misses
> and for files / requests that have a lot hit ratio we don't do "fast reads"
> mostly getting rid of extra latency in the uncached cases.
>
> I've performed other benchmarks and I have no observed any perf regressions in
> any of the normal (old) code paths.
>
>
> I have co-developed these changes with Christoph Hellwig.
>
> Milosz Tanski (4):
>   vfs: Prepare for adding a new preadv/pwritev with user flags.
>   vfs: Define new syscalls preadv2,pwritev2
>   vfs: Export new vector IO syscalls (with flags) to userland
>   vfs: RWF_NONBLOCK flag for preadv2
>
>  arch/x86/syscalls/syscall_32.tbl  |   2 +
>  arch/x86/syscalls/syscall_64.tbl  |   2 +
>  drivers/target/target_core_file.c |   6 +-
>  fs/cifs/file.c                    |   6 ++
>  fs/nfsd/vfs.c                     |   4 +-
>  fs/ocfs2/file.c                   |   6 ++
>  fs/pipe.c                         |   3 +-
>  fs/read_write.c                   | 121 +++++++++++++++++++++++++++++---------
>  fs/splice.c                       |   2 +-
>  fs/xfs/xfs_file.c                 |   4 ++
>  include/linux/aio.h               |   2 +
>  include/linux/fs.h                |   7 ++-
>  include/linux/syscalls.h          |   6 ++
>  include/uapi/asm-generic/unistd.h |   6 +-
>  mm/filemap.c                      |  22 ++++++-
>  mm/shmem.c                        |   4 ++
>  16 files changed, 163 insertions(+), 40 deletions(-)
>
> --
> 2.1.0
>



-- 
Milosz Tanski
CTO
16 East 34th Street, 15th floor
New York, NY 10016

p: 646-253-9055
e: milosz@xxxxxxxxx
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