Memory Organization 2: External Memory

Performance : Access time

Formula untuk cari Access Time:

Cakera Latency = Masa Seek + Masa Putaran + Masa Pemindahan + Pengawal overhed

Masa Seek: bergantung kepada nombor trek bergerak

          “lengan”,  kelajuan seek cakera

Masa Putaran : bergantung kepada cakera kelajuan

             berputar, sejauh mana sektor dari kepala

Masa Pemindahan : bergantung kepada kadar data (bandwidth) cakera (bit density), saiz data yang diminta

Pengawal overhed : 0.1ms

Contoh 1:

Baca 512-byte, purata masa seek = 5ms, kadar pemindahan = 40MB/sec, putar kelajuan = 10000RPM, pengawal overhed = 0.1ms

Purata masa seek = 5ms

Purata masa putaran = 0.5/(10000RPM)

                 = 0.5/(10000RPS/60)

                 = 0.003s

                 = 3ms

Masa pemindahan = 512B/(40MB/s)

               = 512B/(40000000B/1000000ms)

               = 12.8ms

Cakera Latency = Masa Seek + Masa Putaran + Masa Pemindahan +

              Pengawal overhed

            = 5ms + 3ms + 12.8ms + 0.1ms

            = 20.9ms

Contoh 2:

Barracuda 180x

Baca 64KB, masa seek = 7.4ms, kadar pemindahan = 65MB/sec, putar kelajuan = 7200RPM, pengawal overhed = 0.1ms

Masa seek = 7.4ms

Purata masa putaran = 0.5/(7200RPM)

                 = 0.5/(7200RPS/60)

                 = 0.0041667s

                 = 4.1667ms

Masa pemindahan = 64KB/(65MB/s)

               = 64KB/(65000KB/1000ms)

               = 0.9846ms

Cakera Latency = Masa Seek + Masa Putaran + Masa Pemindahan +

              Pengawal overhed

            = 7.4ms + 4.1667ms + 0.9846ms + 0.1ms

            = 12.6513ms

Contoh 3:

UltraStar72

Baca 64KB, 1/3 dipetik masa seek, masa seek = 7.4ms, kadar putaran = 7200rpm, kadar pemindahan = 47KB/ms, pengawal overhed = 0.1ms

Masa seek = (1/3)7.4ms

         = 2.4667ms

Purata masa putaran = 0.5/(7200RPM)

                 = 0.5/(7200RPS/60)

                 = 0.0041667s

                 = 4.1667ms

Masa pemindahan = 64KB/(47KB/ms)

               = 64KB/(47KB/ms)

               = 1.3617ms

Cakera Latency = Masa Seek + Masa Putaran + Masa Pemindahan +

              Pengawal overhed

            = 2.4667ms + 4.1667ms + 1.3617ms + 0.1ms

            = 8.0951ms

Contoh 4:

Disk Parameters

Size pemindahan = 8KB, masa seek = 12ms, kadar putaran = 7200rpm, kadar pemindahan = 4MB/s

Masa seek = 12ms

Purata masa putaran = 0.5/(7200RPM)

                 = 0.5/(7200RPS/60)

                 = 4.1667ms

Masa pemindahan = 8KB/(4MB/s)

               = 8KB/(4000KB/1000ms)

               = 2ms

Cakera Latency = Masa Seek + Masa Putaran + Masa Pemindahan +

              Pengawal overhed

            = 12ms + 4.1667ms + 2ms + 0.1ms

            = 18.2667ms

                                                                                                                                BY : TEE SONG WEI