[Lustre-devel] OSC-OST data path optimization

[Jack David]

On Fri, Feb 24, 2012 at 10:43 AM, Andreas Dilger <adilger at whamcloud.com> wrote:
> On 2012-02-23, at 2:43 AM, Jack David wrote:
>> On Thu, Feb 23, 2012 at 12:49 PM, Jack David <jd6589 at gmail.com> wrote:
>>> On Wed, Feb 22, 2012 at 11:37 PM, Andreas Dilger <adilger at whamcloud.com> wrote:
>>>> On 2012-02-22, at 7:04, Jack David <jd6589 at gmail.com> wrote:
>>>>> I am focusing on the WRITE scenario as of now (i.e. lustre client is
>>>>> writing a file on server). On the OSC side, the descriptor ("desc") is
>>>>> filled in osc_brw_prep_request() function, and the preparation for
>>>>> sending the OST_WRITE request to server (i.e. OST) is carried out (I
>>>>> am not familiar with Portal RPC and its mechanics so currently I am
>>>>> skipping the calls which actually prepares the request).
>>>>>
>>>>> On the OST side, upon receiving the OST_WRITE request, the
>>>>> ost_brw_write function will also start the preparation for the
>>>>> buffers. The function invoked is filter_preprw (and in turn
>>>>> filter_preprw_write) will actually find out the corresponding
>>>>> inode/dentry from the "fid" and prepare the pages in which incoming
>>>>> data can be filled.
>>>>>
>>>>> I noticed that while preparing the pages on OST, there is a check
>>>>> which makes sure that if peer_nid and local nid are same. Is it
>>>>> possible that OST/OSC can use this information and OSC will send the
>>>>> page information in the OST_WRITE request, and OST will put it into
>>>>> page_cache (I am not an expert in linux kernel and not sure if linux
>>>>> kernel allows, but idea is to share the pages instead of copying)?
>>>>
>>>> The difficulty is that the cache on the OSS also has its own pages, so either Lustre will need to do nasty things with the page cache for both the client address space and the server address space, or there has to be a memcpy() somewhere in the IO path.
>>>>
>>>> The best way to handle this would be to set up a special combined OSC-OST module that bypasses the RPC layer entirely, but this would be a lot of work to maintain.
>>>>
>>>> While we have thought about doing this for a long time, one important question is whether this is really a bottleneck. It would be easy to see this by running oprofile to see whether the memcpy() is   consuming all of the CPU.
>>>>
>>>
>>> Thanks for the suggestion. I will carry out oprofile tests and see how
>>> much CPU memcpy consumes.
>>>
>>>> Note that in Lustre 2.2 there are multiple ptlrpcd threads that should allow doing the memcpy() on multiple cores.
>>>>
>>>> It might also be worthwhile to automatically disable data checksums for local OSC-OST bulk RPCs, since this can have a noticeable performance impact if both ends are on the same node.
>>>>
>>>
>>> I tried to look for the checksum in the code and found the
>>> "ksocknal_tunables.ksnd_enable_csum". Is this correct parameter to
>>> enable/disable the checksum? I noticed that it is enabled by default.
>>> I tried to disable it in the transmit path (in ksocknal_transmit()).
>>> But what I noticed is, ksocknal_trasmit only come into picture when
>>> OSC and OST are setup on different nodes. If they both are on the same
>>> node, this path is not taken (and may be loopback code comes into
>>> picture lnet/lnet.lo.c - not sure though). Will checksum be enabled
>>> for the loopback path?
>>
>> Okay, while search for checksum code, I missed the checksum
>> calculation done on OSC/OST bulk data if cli->cl_checksum bit it set.
>> This was actually caught up during oprofile report.
>
> Right, that is what I was referring to.
>
>> Following is the top entries in the report.
>
> What version of Lustre is this?

Okay, I do not remember that, because I cloned the git a while back.
Here is the git log shows as the top commit.
-------------------------------------------------------
commit 48452fbe583cf365d3c1f5be3c4272d30e198781
Author: Bobi Jam <bobijam at whamcloud.com>
Date:   Thu Oct 27 09:51:39 2011 +0800

    LU-508 ldiskfs: fix race in ext4_ext_walk_space()
-------------------------------------------------------

>
>> samples  %        app name                 symbol name
>> 2251      7.9462  vmlinux                  copy_user_generic_string
>
> This is copy from userspace.  This is unavoidable for buffered IO,
> though O_DIRECT will help if you are doing very large IO (16-32MB or more).
>
>> 2059      7.2684  ost.ko                   ost_checksum_bulk
>> 1759      6.2094  osc.ko                   osc_checksum_bulk
>
> Like I thought - running client and server on the same node is expensive.
> Turning bulk checksums off for local OSCs would eliminate both of these
> functions from local IO operations.
>
> First thing to verify is if disabling checksums gives any performance
> benefit at all, besides reducing the CPU overhead:
>
> client# lctl set_param osc.*.checksums=0
>
> and re-run your test.
>
> It looks reasonable to clear the cli->cl_checksum at connection time if
> the OST connection is local, if the above shows improvement, something like:
>
> client_connect_import()
> {
>        if (data) {
>                /* disable checksums for local OSTs to avoid double overhead */
>                if (imp->imp_connection->c_peer == imp->imp_connection->c_self)
>                        cli->cl_checksum = 0;
>
>

Yes, I did not know this path, so I cli->cl_checksum flat in
osc_brw_prep_request() (which is not advisable I think as it comes
into I/O path) function which worked for me. And I did not see the
checksum symbols in next oprofile run.

>> 1104      3.8972  vmlinux                  hpet_readl
>> 1007      3.5548  vmlinux                  memcpy_c
>
> This is the memcpy() for the "network" transfer (handled inside LNET).
> This would be eliminated by doing crazy things with the page cache, but
> invalidating page tables and forcing direct-IO on the client may not be
> very efficient either.
>

Right, I think this memcpy happens in lolnd_recv(), CMIIW, I just want
to confirm it.

Thanks for you time. It really helped.
J