[Haifux] SSD and linux

Thu Sep 18 19:29:04 MSD 2008

Hi Gabi,

First of all thanks for you're help.

We are currently using kernel 2.6.24, and couldn't find any call to the
function "block_wait_queue_running(q)" there. It seems to handle things a
bit differently.

Moreover I looked at the code of kernel 2.6.11 and from what I can
understand, it seems to me like the "block_wait_queue_running(q)" function
only waits on the IO queue for the IO to be ready, and not for the IO
request (reading the page from the disk) to be done (it is called before the
IO request is made).

Correct me if I'm wrong but isn't the call to the function
"q->make_request_fn(q,bio)" what makes the actual request to the device, and
therefore the place which is responsible for waiting for the result of that
request?

P.S.
I don't mind switching to kernel 2.6.11 (or any other for that matter) as
long as I can make the changes I need.

Thanks,
Ronen & Doron

On Thu, Sep 18, 2008 at 4:49 PM, gabik <gabik at cs.technion.ac.il> wrote:

>  Hi Doron
>
> The place where the current process goes to sleep and waits until the page
> is swapped in is indeed in generic_make_request() (called from submit_bio())
> There is a call to block_wait_queue_running(q); which moves this process to
> wait and calls for schedule() [prepare_to_wait_exclusive() and after that
> io_schedule()].
> Thus, this seems to be a place for a busy loop.
>
> You must be careful though with what you change and make sure not to break
> some other code path, that assumes certain things done in this code path.
> For example, if you are not going to put this process in the wait queue,
> you must be careful what will happen when the io operation will finish and
> will want to remove this process from the wait queue and wake it up.
>
>
> Gabi
>
> P.S. I was referring to version 2.6.11
> http://lxr.linux.no/linux+v2.6.11/drivers/block/ll_rw_blk.c#L2595
>
>
>  ------------------------------
> *From:* Doron Zuckerman [mailto:doron.zuckerman at gmail.com<https://mail.google.com/mail?view=cm&tf=0&to=doron.zuckerman@gmail.com>]
>
> *Sent:* Thursday, September 18, 2008 12:28 PM
> *To:* muli at il.ibm.com<https://mail.google.com/mail?view=cm&tf=0&to=muli@il.ibm.com>;
> gabik at cs.technion.ac.il<https://mail.google.com/mail?view=cm&tf=0&to=gabik@cs.technion.ac.il>;
> haifux at haifux.org<https://mail.google.com/mail?view=cm&tf=0&to=haifux@haifux.org>
> *Subject:* Re: [Haifux] SSD and linux
>
>  Hi Gabi and Muli,
> I'm sorry about the mistake- you understood me correctly.
>
> I'm not sure it will speed up the OS, however I'm doing an academic
> research on the matter as part of a project I'm taking, and I plan to check
> this point.
> The leading thought was that since the SSD is not a mechanical drive, pages
> can be brought faster in this way, and there is no need to context switch,
> thus, avoiding the overhead included.
>
> Yes I plan to use the polling system (busy-wait) , and I'm looking for the
> kernel part  in the pagefault handling mechanism in which the process is
> suspended in order to prevent it.
>
> So far I found the function "__generic_make_request" in file "ll_blk".
> This function calls a sub function named "might_sleep".
> I have deleted the call to this function whenever I'm in a pagefault,
> however I'm not sure if this function casuses the sleep, or is just used for
> debugging in order to check if we entered a suspend state.
>
> My question is if this is the function I should change in order to accept
> the change I'm willing to get, or if the change should be made in
> q->make_request_fn
> which, according to my understanding, belongs to the specific driver I'm
> using.
>
> Please help me find the specific place I'm looking for that would make the
> desired change.
>
> Thank you very much,
> Doron.
>
> On Tue, Sep 16, 2008 at 2:42 PM, gabik <gabik at cs.technion.ac.il<https://mail.google.com/mail?view=cm&tf=0&to=gabik@cs.technion.ac.il>
> > wrote:
>
>>  Hello Doron
>>
>> Why do you think it will speed up the OS?
>> What do you plan to do until the page is swapped in? Busy loop?
>>
>> About your solution:
>> handle_mm_fault is called from within page fault handler (do_page_fault<http://lxr.linux.no/linux+v2.6.26.5/+code=do_page_fault>
>> ()).
>> So what is the rational behind calling handle_mm_fault not from inside
>> pagefault  handler?
>> Where would you call it from instead and what do you plan to do when you
>> are in the page fault?
>>
>> Probably what you meant is, in order not to do context switch due to page
>> fault, is to call handle_mm_fault as usual, but not to raise need_resched
>> flag, so as not to trigger a context switch in case of a major page fault.
>>
>>
>> Gabi
>>
>>
>>
>>  ------------------------------
>> *From:* haifux-bounces at haifux.org<https://mail.google.com/mail?view=cm&tf=0&to=haifux-bounces@haifux.org>[mailto:
>> haifux-bounces at haifux.org<https://mail.google.com/mail?view=cm&tf=0&to=haifux-bounces@haifux.org>]
>> *On Behalf Of *Doron Zuckerman
>> *Sent:* Tuesday, September 16, 2008 12:31 PM
>> *To:* haifux at haifux.org<https://mail.google.com/mail?view=cm&tf=0&to=haifux@haifux.org>
>> *Cc:* Ronen Gruengras
>> *Subject:* [Haifux] SSD and linux
>>
>>   Hi all,
>>
>> I have a question regarding the linux kernel (for those of you who are
>> familiar with it).
>>
>> I'm looking for a way to add a change to the linux kernel in order to
>> check if I can make it more compatible with my Asus EEE-PC.
>> I would like to change the kernel in such way that it will not do a
>> context switch every time there is a page fault
>> and will wait for the required page to be brought from the SSD (Solid
>> State Drive), then continue as usual.
>> In Such way, I plan to check if I can fasten the speed of the Operating
>> System (Ubuntu for EEE).
>> I thought of adding a TIF flag in the process descriptor
>> (thread_info_32.h) that will tell me if I'm currently in a pagefault and
>> then change the fault_32.c in such way that it will do the
>> "handle_mm_fault(mm,vma, address, write_;" only if there is no
>> pagefault at the moment.
>> Can you suggest any other solution possible or tell me what you think
>> about this solution.
>>
>> I would really appreciate any help with this,
>> Doron.
>>
>>
>>
>
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