RE: Better choice for Linux semaphore than spinlock?

[Krotil, Radek]

From what I understand, the svnserve on Windows can run only in the threaded 
mode. On Linux the default is the fork mode, but using the fork mode may 
produce significant overhead and excessive memory allocation due to the caches 
at high concurrency. So this particular case where the problem was reported is 
coming likely from a threaded svnserve.

When it comes to comparison of between Windows and Linux deployment, our 
experience shows that svnserve stalling happens sooner on Windows than on Linux 
using fork mode. Symptoms include all CPU being burned in svnserve process and 
long response times in terms of 100 seconds on SVN side. Note that our 
application in parallel uses http connection to get and put data into SVN on 
behalf of regular end users, and also system user access via svn protocol and 
svnserve. Both Apache and Svnserve are running on top of the same repository.

This subject has been opened some 18 months ago as well, and you can see the 
history of the conversation at 
http://subversion.1072662.n5.nabble.com/Deadlock-like-behaviour-of-svnserve-in-multi-threaded-mode-T-td196421.html#a197955.

I can pull in additional experts from our team that were involved in the 
analysis with Red Hat in detail.

Best regards,
Radek Krotil

Siemens Digital Industries Software
Polarion ALM Product Management
polarion.plm.automation.siemens.com<https://polarion.plm.automation.siemens.com/>


From: Nathan Hartman 
Sent: Tuesday, February 4, 2020 12:55 PM
To: Krotil, Radek (DI SW LCS PMT ALM) ; Subversion 
Developers 
Subject: Re: Better choice for Linux semaphore than spinlock?

On Tue, Feb 4, 2020 at 6:45 AM Krotil, Radek 
mailto:radek.kro...@siemens.com>> wrote:
Hi All.
I believe this issue originates at one of our customer and is related how 
Polarion ALM is using Subversion at scale. This is a reoccurring issue being 
encountered by several enterprise customers and I’d be more than happy to help 
the community to pin it down and get it fixed. Has there been any update since 
October on this problem?

I think there haven't been any changes in this area but we will definitely 
appreciate your help.

The underlying cause might be in the APR (Apache Portable Runtime) libraries, 
which Subversion uses for its platform independence, or could be in the choice 
of APR APIs used somewhere in svnserve.

By the way, are you doing a threaded or non-threaded build?

Nathan


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Re: Better choice for Linux semaphore than spinlock?

[Krotil, Radek]

Hi All.
I believe this issue originates at one of our customer and is related how 
Polarion ALM is using Subversion at scale. This is a reoccurring issue being 
encountered by several enterprise customers and I'd be more than happy to help 
the community to pin it down and get it fixed. Has there been any update since 
October on this problem?
On the customer end the problem is easy to detect, where almost all CPU is 
consumed by svnserve process, while more than 95% of the CPU is spent in system 
time, leaving almost no throughput for the actual operation.
[cid:image001.png@01D5DB52.DAB45430]
Best regards,
Radek Krotil

Siemens Digital Industries Software
Polarion ALM Product Management
polarion.plm.automation.siemens.com


On 2019/10/12 23:29:44, eponymous alias  wrote:
>  Perhaps these links might be of help in some way:>
>
> https://webkit.org/blog/6161/locking-in-webkit/>
> https://blog.mozilla.org/nfroyd/2017/03/29/on-mutex-performance-part-1/>
> https://preshing.com/2018/locks-arent-slow-lock-contention-is/>
>
> On Monday, October 7, 2019, 1:56:14 PM PDT, Doug Robinson 
>  wrote:>
>
> Rüdiger:>
>
> On Mon, Oct 7, 2019 at 3:51 PM Ruediger Pluem  wrote:>
>
>  On 10/07/2019 08:40 PM, Branko Čibej wrote:>
>  > On Mon, 7 Oct 2019, 19:47 Doug Robinson,  > wrote:>
>  >>
>  > Folks:>
>  >>
>  > I spoke with this user late last week. They stated that they can only get 
> approximately 400 parallel SVN operations>

>  > before the "system time" consumes all available CPU for an 8-core machine. 
> Adding more cores won't help because of>

>  > the nature of spin locks (it makes things worse). Turns out that even with 
> ~100 parallel SVN operations the "system>

>  > time" starts becoming significant/measurable (~10%). Both HTTP 
> (mod_dav_svn) and "svnserve" protocols participate>

>  > in the lock contention.>
>  >>
>  > Your help would be greatly appreciated.>
>  >>
>  > Whew. So. Reducing this issue to "use a more efficient lock" is not going 
> to work, and you provided far too little>

>  > information to even attempt a diagnosis. For starters, I recommend 
> gathering as much info as possible (anonymised of>

>  > course) about the server configuration, everything from httpd an svnserve 
> to the repository config and underlying>

>  > filesystem, if possible. Getting stack traces of the "stuck" threads would 
> be necessary, too. Without knowing exactly>

>  > what is happening, these kinds of problems are extremely hard to 
> understand, let alone fix.>
>
>  Plus depending on which part of the code requires this lock a different 
> locking mechanism that might suit better for>

>  this use case can possibly be chosen via configuration changes (e.g. httpd 
> allows this for most of its locking).>
>
> That would be awesome! I'll definitely try to get those stack tracebacks.>
>
> Cheers.>
>
> Doug>
>   >

-
Siemens Industry Software, s.r.o.
Praha 4, Doudlebská 1699/5, PSČ 140 00
IČ 256 51 897
Zapsaná v obchodním rejstříku vedeném Městským soudem v Praze, oddíl C, vložka 
58222

Důležité upozornění: Tato zpráva má jen informativní charakter. Obsah této 
zprávy odesílatele nezavazuje a odesílatel nemá v úmyslu touto zprávou uzavřít 
smlouvu, přijmout nabídku, potvrdit uzavření smlouvy ani nezakládá předsmluvní 
odpovědnost jejího odesílatele, ledaže je odesílatelem ve zprávě uvedeno 
výslovně jinak. Obsah této zprávy (včetně příloh) je důvěrný. Pokud nejste 
zamýšleným adresátem této zprávy, zpřístupnění, kopírování, distribuce nebo 
užití obsahu zprávy je přísně zakázáno a v takovém případě, prosím, okamžitě 
informujte odesílatele a poté zprávu (vč. příloh) odstraňte z Vašeho systému.

Important Note: This message is only of informative nature. The content of this 
message shall not be binding for sender and sender does neither intend to 
conclude contract, accept offer or confirm the conclusion of the contract by 
this message nor this message represents pre-contractual liability of the 
sender, unless the sender states in the message excplicitly otherwise. The 
content of this message (including appendices) shall be confidential. Should 
you are not intended receiver of this message, any access, copying, 
distribution or use of the content of this message is strictly prohibited and 
in such case, please immediately notify the sender and subsequently delete the 
entire message (including apppendices) from your system.