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Date: Sat, 22 Sep 2012 11:35:34 +0530 From: anish singh <anish198519851985@...il.com> To: Daniel Taylor <Daniel.Taylor@....com> Cc: Deepawali Verma <dverma249@...il.com>, Tejun Heo <tj@...nel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: Work queue questions On Sat, Sep 22, 2012 at 10:57 AM, Daniel Taylor <Daniel.Taylor@....com> wrote: > > >> -----Original Message----- >> From: linux-kernel-owner@...r.kernel.org >> [mailto:linux-kernel-owner@...r.kernel.org] On Behalf Of anish singh >> Sent: Friday, September 21, 2012 9:25 PM >> To: Deepawali Verma >> Cc: Tejun Heo; linux-kernel@...r.kernel.org >> Subject: Re: Work queue questions >> >> On Sat, Sep 22, 2012 at 1:05 AM, Deepawali Verma >> <dverma249@...il.com> wrote: >> > Hi Tajun, >> > >> > These three tasks are writing the three chunks of data in >> parallel. I >> > am not getting improvement here otherwise what is difference between >> > writing these chunks one by one in single thread instead of >> trying to >> > write the data by scheduling the work on three different workqueues >> > means 3 worker threads? >> You should have carefully read "If none of them blocks, there >> isn't much point in throwing more threads at them. What are those >> thread doing?" what Tejun said. >> >> I think what he means is that concurrency is the concept of >> keeping the >> system busy. >> If you see the below logs: >> kworker/u:1-21 [000] 110.964895: task_event: >> MYTASKJOB2381 XStarted >> kworker/u:1-21 [000] 110.964909: task_event: >> MYTASKJOB2381 Xstopped >> Here your first worker thread blocked. >> >> So the system will try to get other workqueue started which is: >> kworker/u:1-21 [000] 110.965137: task_event: >> MYTASKJOB2382 XStarted >> kworker/u:1-21 [000] 110.965154: task_event: >> MYTASKJOB2382 Xstopped >> Here again your second worker thread blocked. >> >> So on so forth. >> Anyway how can you write chunks of data in parallel when >> already some worker >> thread is writing i.e. the system is busy. >> Analogy: Suppose you are ambidextrous and you are eating.Can >> you eat with >> both of your hands at a time?So worker thread are like your >> hands and keeping >> you fed all the time is the concept of concurrency. >> >> I am not an expert on this but from Tejun's reply I could >> make out this. >> Please correct me If I have wrongly understood the concept >> based on this mail > > I don't know how many cores are in the CPU Deepawali's using, but if I have four, Assuming single core,Is my explanation correct about concurrency? > for example, I could do something simplistic like copy pages A-G with one, pages > H-O with another, and pages Q-Z with a third. There are memory and cache bottlenecks > (like the mouth, in your example), but all three copies could be running concurrently. > > Copying, of course, is a silly, trivial example, and I hope there's a better reason > than that for the concurrency, but, if, for example, your needed to byte-swap, XOR, > or checksum, as core functionality of an embedded system, and the processing units were > available to do these things in parallel, then interleaving those operations with memory > accesses could provide higher throughput. > > I think what he's asking is why there's no apparent concurrency, presuming that NONE > of his threads has a real reason to block. With examining his code, I cannot tell, > but it looks like, from the messages, that the kernel did not attempt concurrency. > Perhaps he needs to pass additional state to the scheduler? > > >> chain. >> > >> > Regards, >> > Deepa >> > >> > On Fri, Sep 21, 2012 at 8:27 PM, Tejun Heo <tj@...nel.org> wrote: >> >> On Fri, Sep 21, 2012 at 08:26:01PM +0100, Deepawali Verma wrote: >> >>> kworker/u:1-21 [000] 110.964895: task_event: >> MYTASKJOB2381 XStarted >> >>> kworker/u:1-21 [000] 110.964909: task_event: >> MYTASKJOB2381 Xstopped >> >>> kworker/u:1-21 [000] 110.965137: task_event: >> MYTASKJOB2382 XStarted >> >>> kworker/u:1-21 [000] 110.965154: task_event: >> MYTASKJOB2382 Xstopped >> >>> kworker/u:5-3724 [000] 110.965311: task_event: >> MYTASKJOB2383 XStarted >> >>> kworker/u:5-3724 [000] 110.965325: task_event: >> MYTASKJOB2383 Xstopped >> >>> >> >>> I have this one big task to whom I divided into small sub >> tasks. These >> >>> are numbered 2381, 2382 and 2383, what was I expecting >> that task 2381, >> >>> 2382, 2383 run in parallel. I have put start and stop >> markers here so >> >>> that I can see how this concurrency managed work queue is >> distributing >> >>> the load. >> >>> >> >>> I found that task no 2381 is started first and exited >> before starting >> >>> task 2382 and so on. What I expected that it should start >> the three >> >>> sub tasks in parallel, not one by one. >> >>> >> >>> Where is concurrency here? >> >> >> >> If none of them blocks, there isn't much point in throwing more >> >> threads at them. What are those thread doing? >> >> >> >> Thanks. >> >> >> >> -- >> >> tejun >> > -- >> > To unsubscribe from this list: send the line "unsubscribe >> linux-kernel" in >> > the body of a message to majordomo@...r.kernel.org >> > More majordomo info at http://vger.kernel.org/majordomo-info.html >> > Please read the FAQ at http://www.tux.org/lkml/ >> -- >> To unsubscribe from this list: send the line "unsubscribe >> linux-kernel" in >> the body of a message to majordomo@...r.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> Please read the FAQ at http://www.tux.org/lkml/ >> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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