Hello Andreas,
The above patch doesn't affect my scenario on a single rank. However, i
suspect there is a bug in the FR-FCFS implementation in gem5.
Looks like, the requests are not overlapping. Precharge and/or activate has
to be issued once a memory request entered the queue. So in a queue, if
there are few ahead read requests and few following
read requests all targeting to a same row in Bank0 to a read request which
targets Bank1 and not a row hit. Worst case delay for Bank1 request should
be tRP+rRCD. After this delay Bank1 request should also become a row hit
(after the precharge and activate). However according to current
implementation the request to Bank1 is served only after all the ahead and
following row hits are done, as these timing parameters are set in
doDRAMAccess(), which is called after reordering the queue.
Is this observation looks correct?
Thanks,
Prathap
On Fri, Nov 14, 2014 at 8:45 AM, Andreas Hansson <andreas.hans...@arm.com>
wrote:
> Hi Prathap,
>
> It would very well be a bug. We have got a small fix for the FRFCFS
> about to go out (but it should not affect your scenario assuming It is a
> single rank):
>
> diff --git a/src/mem/dram_ctrl.cc b/src/mem/dram_ctrl.cc
> --- a/src/mem/dram_ctrl.cc
> +++ b/src/mem/dram_ctrl.cc
> @@ -1483,8 +1489,8 @@
> // 1) Commands that access the same rank as previous burst
> // and can prep the bank seamlessly.
> // 2) Commands (any rank) with earliest bank prep
> - if (!switched_cmd_type && same_rank_match &&
> - min_act_at_same_rank <= min_cmd_at) {
> + if ((bank_mask == 0) || (!switched_cmd_type && same_rank_match &&
> + min_act_at_same_rank <= min_cmd_at)) {
> bank_mask = bank_mask_same_rank;
> }
>
> Could you give that a spin?
>
> Thanks,
>
> Andreas
>
> From: Prathap Kolakkampadath via gem5-users <gem5-users@gem5.org>
> Reply-To: Prathap Kolakkampadath <kvprat...@gmail.com>, gem5 users
> mailing list <gem5-users@gem5.org>
> Date: Friday, 14 November 2014 00:11
> To: gem5 users mailing list <gem5-users@gem5.org>
> Subject: [gem5-users] DRAMCTRL:Seeing an unusual behaviuor with FR-FCFS
> scheduler
>
> Hi Users,
>
> For the following scenario:
>
>
> Read0 Read1 Read2 Read3 Read4 Read5 Read6 Read7 Read8 Read9 Read10 Read11
>
> There are 12 reads in the read queue numbered in the order of arrival.
> Read 0 to Read3 access same row of Bank1, Read4 access Bank0, Read5 to
> Read8 access same row of Bank2 and Read9 to Read11 access same row of Bank3.
>
> According to FR-FCFS scheduler, even there is only a single request
> Read4 to Bank0, it should be scheduled after the Read0 to Read3 are
> scheduled. Because within the window of Read0-Read3, the Read4 would have
> done with precharge and activate and ready to schedule. Though Read5 and
> Read9 are also ready, Read4 needs to be scheduled as the next row hit,
> according to FCFS.
>
> However i see a different behaviour where Read4 is scheduled only after
> all other row hits to Bank2 and Bank3 is scheduled. Also i noticed from
> the debug prints that Read4 is not becoming a row_hit.
>
> Are we missing to mark the read as row hit after precharge and activate.
> I am trying to figure this out. Is my understanding correct?
>
> Thanks,
> Prathap
>
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