حافظه کش یا حافظه نهان درون پردازنده چیست؟
حتما تاکنون بارها و بارها نام Cache Memory ، حافظه کش و یا حافظه پنهان و سطح بندی های مختلف آن را در توضیحات و بررسی خصوصیات پردازنده ها شنیده اید، ما در این بخش قصد داریم به طور کامل به این مموری پرسرعت بپردازیم. با طیف نیوز همراه باشید… .
تعریف کلی
حافظه کش یک حافظه نهان بسیار سریع SRAM است که مانع از ایجاد تأخیر بین حافظه رم که از نوع DRAM است و CPU می شود. این مموری حدود چهل سال است که استفاده می شود. به بیان ساده تر Cache یک حافظه پنهان کوچک است که همیشه برخی اطلاعاتی که در حافظه رم وجود دارند را در خود ذخیره می کند، در نتیجه پردازنده خیلی سریع میتواند این داده ها را دراختیار داشته باشد.
یک مثال ساده و بیانی واضح و مفهومی
یک کتابخانه را درنظر بگیرید. در این کتابخانه کسی که کتاب درخواستی را از میان انبوه کتاب ها می آورد پردازنده، و فضای کل کتاب خانه با هزاران کتابی که دارد حافظه رم است. زمانی که شما سفارش کتاب می دهید متصدی کتابخانه باید مسیری را طی کند و زمانی را بگذراند تا به قسمت قفسه ها رفته و کتاب را بردارد و دوباره آن مسیر را برگردد و به شما تحویل دهد این کار را پردازنده انجام می دهد اما اگر چند کتاب پرطرفدار و معروف را در به اتاق مدیریت ( حافظه پنهان ) بیاورد و گلچین کند تایم کمتری طی می شود تا شما به کتابتان دسترسی پیدا کنید.
این اتاق که در آن کتاب های گلچین شده (یعنی همان اطلاعات ذخیره شده) قرار میگیرند، همان حافظه کش یا حافظه نهان است که سرعت دسترسی به اطلاعات را برای پردازنده و کاربر بسیار بالا می برد.
علت روی کار آمدن این مموری پرسرعت
پردازنده ها فعالیت خود را با دستوری که از سیستم عامل نصب شده روی هارد، دریافت می کنند، اجرا مینمایند. از طرفی برنامه ها و اطلاعات مختلف روی هارد ذخیره می شوند اما دسترسی به این اطلاعات در هارد زمان گیر است، در نتیجه در این میان یک حافظه RAM طراحی شده است تا داده ها را از هارد گرفته و اینگونه به انتقال اطلاعات سرعت دهد و به پردازنده آنها را برساند. اما همچنان اتلاف تایم وجود دارد، برای افزایش این سرعت انتقال اطلاعات مموری Cache طراحی شده و بین رم و پردازنده قرار داده شده است.
اما این پایان راه پرسرعت انتقال اطلاعات نیست. پس از از حافظه نهان ثبات ها روی کار آمدند، ثبات ها با سرعت بسیار بیشتری که نسبت به حافظه پنهان دارند برای افزایش سرعت میان حافظه کش و پردازنده استفاده می شوند.
اسمارت کش چیست؟
قبل از ریو کار آمدن حافظه کش هوشمند ، اسمارت کش یا همان حافظه پنهان اشتراکی، در برای هر هسته یک Cache Memory با حجم مشخص در نظر میگرفتند که در چنین شرایطی اگر یکی از هسته ها به فضای ذخیره سازی بیشتر نیاز پیدا می کرد به حافظه رم ارجاع داده می شد. اما حالا دیگر با وجود حافظه نهان هوشمند یک حجم معین وجود دارد تا هر هسته طبق آنچه نیاز دارد از این فضا استفاده کند. یعنی دیگر احتیاجی به رجوع به رم و اتلاف تایم نخواهد بود.
یک مثال ساده و بیاین واضح و مفهومی
فرض کنید چهار هسته در پردازنده داشته باشیم. هسته دو مسئول پردازش کارهای فتوشاپی و هسته سه مسئول پردازش کارهای media player باشد. در این حالت هسته دو به حافظه کش بیشتری نیازمند است و هسته سه کمتر. در نتیجه هفتاد درصد از کش هوشمند و یا اشتراکی به هسته دو و مابقی بین سه هسته دیگر تقسیم می شود. البته این تا زمانی است که فتوشاپ انجام میگیرد، پس از آن، وقتی هسته دو دیگر نیازی به حافظه پنهدن نداشته باشد، دوباره این تقسیم بندی ها تغییر می کند.
سطح بندی
سطح بندی موضوع پیچیده ای نیست. خیلی ساده آنرا در این بخش از طیف نیوز معرفی خواهیم کرد. تصور کنید یک پردازنده فقط یک Cache داشته باشد. زمانی که cpu داده های مورد نیاز خود را در این حافظه پنهان پیدا نکند ناچارا باید به رم مراجعه کند اما این امر زمانگیر است. برای پیشگیری از این مشکل یک حافظه نهان ذخیره و یا زاپاس می تواند به دسترسی اطلاعات سرعت بخشد.
حالا آن مموری که به پردازنده نزدیکتر باشد حافظه کش سطح یک و به ترتیب فاصله از پردازنده دو، سه و چهار نامیده می شود. چنانچه اطلاعات در سطح یک نبود از سطح سه و اگر در سطح سه نیز نبود از سطح چهار اما به هر حال چنانچه هیچ اطلاعاتی از این نقل و انتقالات در سطح ها عاید نشود اصطلاحا می گویند Cache miss رخ داده و اگر داده ها به دست آیند می گویند Cache hit شده. اطلاعات برداشته می شوند. این سرعتر از رجوع مستقیم به رم می باشد. درکل اگر در سطح چهار هم خبری از داده مورد نظر نبود و کش میس سد، به رم و اگر نبود به هارد رجوع داده می شود.
نکات
هر چه سطح حافظه کش بیشتر شود سرعت کمتر و حجم بیشتر می شود. اما در کل هر سه سطح حافظه کش از حافظه رم سرعت بیشتری دارند.
در ارتباط با کش میس و کش هیت هر چه معماری طراحی پردازنده ها قوی تر باشد کش هیت بیشتر است و درصد بالاتری دارد. برای مثال در cpu های پرسرعت و بالارده Cache hit 99 الی 100 درصد می باشد.
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