Question(s) about smbd in respect to preadv2

[Milosz Tanski]

On Sat, Jan 24, 2015 at 6:39 PM, Jeremy Allison <jra at samba.org> wrote:
> On Sat, Jan 24, 2015 at 03:46:18PM -0500, Milosz Tanski wrote:
>> I'm at a bit of a cross roads with testing preadv2 using samba and how
>> samba could benefit from the syscall. I need to better understand the
>> samba architecture today and where it's going. I spent a few hours
>> yesterday and today better to understand samba; I also looked at
>> Jermey's SambaXP 2014 talk. There's a few clarifications I need.
>>
>> I did some preliminary testing using preadv2 to perform. The first
>> tests I ran were using the source3 smbd server. And I compared the
>> sync, pthreadpool and pthreadpool + preadv2. Just a simple rand read
>> test with one client with a cached data.
>>
>> The results were that sync was fastest (no surprise there).
>> Pthreadpool + preadv2 was about 8% slower then sync. Plain old
>> pthreadpool 26% slower. So not a bad win there. Additionally, it looks
>> like the vfs_pread_send and vfs_pread_recv have a bit more overhead
>> over plain old vfs_pread in the code path so it's possible to get that
>> 8% even closer to the sync case.
>>
>> So far, so good... but what struct me is that I don't really
>> understand why samba uses the pthreadpool if forks() for each client
>> connection? Why bother.
>
> Because it allows a client to have multiple outstanding
> read and write IOops to a single smbd daemon.
>
> This is important if a client has multiple processes
> reading and writing multiple large files - the client
> redirector just pipelines to the number of outstanding
> SMB2 credits.
>
> Using pthreadpool then allows a single smbd to have
> multiple preads/pwrites outstanding.

I can only imagine that the latency distribution would shift left
(lower) for fully cached / sequential reads as I've seen that in our
app that developed preadv2 for originally.

I haven't figured out yet to make async client submit multiple SMB2
async requests using the cifs FIO engine yet After spending more time
reading the stuff in libcli, it looks like the code is there it just
not exported in a external library.

-- 
Milosz Tanski
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