Re: SMC for page crossing traces

Hi,

This optimization is already long time in my mind but I had no time to try
it and see the impact.

The positive side is trivial - it would eliminate a need to decWriteStamp
for all writes which do not share code in the same 4K page.
But the other hand, when false sharing occurs (data and code are located in
the same page) - the emulation performance will be terrible because all
stores would have to go slow path.
And not only stores, also code fetches ...
We have observed huge amount of false sharing in Win95/98 and also not so
small amount in WinXP code.
Making SMC fine grained (128-byte granularity) had really visible impact on
Windows boot emulation performance.

Need to check if gain from all the "good" stores is bigger than loss from
false sharing cases and see which one is better.

As possible solution Darek suggested to implement L0TLB where an "entry" has
a 128-byte length "pages" only with all attributes as for normal 4K pages.
This way code crossing 128-byte boundary will be considered sort of "page
split" and need to be treated special way.
But from other hand your idea could be implemented on lower than 4K
granularity and avoid false sharing problem.

Another issue to consider here - multiprocessor. The pageWriteStamp table is
unified while TLB is per-processor.
So fetch from a page in one processor would cause chain of TLB invalidation
on all others. This problem doesn't exists if emulating single CPU only.

Not sure I will have time to deal with it soon anyway ... would you like to
try to provide a patch ?

Stanislav

-----Original Message-----
From: Xin Tong [mailto:[email protected]] 
Sent: Wednesday, October 31, 2012 5:21 PM
To: Stanislav
Cc: [email protected]
Subject: Re: [Bochs-developers] SMC for page crossing traces

another optimization for SMC is not to installed pages that have code
translated from into the TLB with write permission. later a write will miss
the TLB and BOCHS can take a longer path to handle that (i.e.
call decWriteStamp). this will eliminate the need to call decWriteStamp on
every write.

Xin


On Thu, Oct 25, 2012 at 1:23 AM, Stanislav <[email protected]> wrote:
> Yes, it is already changes in SVN. Including few more observations 
> that you didn't see yet .
>
>
>
> Unfortunately SVN services are not available right now for Bochs. 
> Might be you could look on latest SVN snapshot that Volker put on the
Bochs webpage.
>
>
>
> Stanislav
>
>
>
> From: Xin Tong [mailto:[email protected]]
> Sent: Thursday, October 25, 2012 1:01 AM
> To: [email protected]
> Subject: [Bochs-developers] SMC for page crossing traces
>
>
>
> I am reading this code and wondering why the entry is not invalidated 
> when the second part of the trace is written into.
>
>
>
>  if (mask & 0x1) {
>
>     // the store touched 1st cache line in the page, check for
>
>     // page split traces to invalidate.
>
>     for (i=0;i<BX_ICACHE_PAGE_SPLIT_ENTRIES;i++) {
>
>       if (pAddr == pageSplitIndex[i].ppf) {
>
>         pageSplitIndex[i].ppf = BX_ICACHE_INVALID_PHY_ADDRESS;
>
>         // SHOULD pageSplitIndex[i].e BE INVALIDATED HERE?
>
>       }
>
>     }
>
>   }
>
>
>
> Xin
>
>


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